of Nebraska, as Paramylodon; it has only four pairs of teeth, and an elongate skull with an inflated muzzle. All the above genera differ from Megatherium in having a foramen on the inner side of the lower end of the humerus. A presumed large ground-sloth from Madagascar has been described, on the evidence of a limb-bone, as Bradytherium, but it is suggested by Dr F. Ameghino that the specimen really belongs to a lemuroid. Be this as it may, the North American mammals described as Moropus and Morotherium, in the belief that they were ground-sloths, are really referable to the ungulate group Ancylopoda.
Although a few of the Pleistocene ground-sloths, such as Nothropus and Nothrotherium (＝Coelodon), were of comparatively small size, in the Santa Cruz beds of Patagonia few of the representatives of the family much exceeded a modern sloth in size. The best-known generic types are Eucholoeops, Hapalops and Pseudahapalops, of which considerable portions of the skeleton have been disinterred. In these diminutive ground-sloths the crowns of the cheek-teeth approached the prismatic form characteristic of Mega[lo]therium, as distinct from the subcylindrical type occurring in Mylodon, Glossotherium, &c.
By many palaeontologists a group of North American Lower Tertiary mammals, known as Ganodonta, has been regarded as representing the ancestral stock of the ground-sloths and those of other South American edentates; but according to Professor W. B. Scott this view is incorrect and there is no affinity between the two groups. If this be so, we are still in complete darkness as to the stock from which the South American edentates are derived.
See W. B. Scott, Mammalia of the Santa Cruz Beds, Edentata, Rep., Princeton Exped. to Patagonia, vol. v. (1903–1904); B. Brown A New Genus of Ground-Sloth from the Pleistocene of Nebraska, Bull. Amer. Mus. Nat. Hist., xix, 569 (1903). (R. L.*)
MYLONITE (Gr. μυλών, a mill), in petrology, a rock which has been crushed and ground down by earth movement and at the same time rendered compact by pressure. Mylonites are fine-grained, sometimes even flinty, in appearance, and often banded in parallel fashion with stripes of varying composition. The great majority are quartzose rocks, such as quartzite and quartz-schist; but in almost any type of rock mylonitic structure may be developed. Gneisses of various kinds, hornblende-schists, chlorite-schists and limestones are not infrequently found in belts of mylonitic rock. The process of crushing by which mylonites are formed is known also as “granulitization” and “cataclasis,” and mylonites are often described as granulites, though the two terms are not strictly equivalent in all their applications. Mylonites occur in regions where there has been considerable metamorphism. Thrust planes and great reversed faults are often bounded by rocks which have all been crushed to fine slabby mylonites, that split readily along planes parallel to the direction in which movement has taken place. These “crush-belts” may be only a few feet or several hundred yards broad. The movements have probably taken place slowly without great rise of temperature, and hence the rocks have not recrystallized to any extent.
Crushing and movement on so extensive a scale are to be expected principally in regions consisting of rocks greatly folded and compressed. Hence mylonites are commonest in Archean regions, but may be found also in Carboniferous and later rocks where the necessary conditions have prevailed. Within a short space it is often possible to trace rocks from a normal to a highly mylonized condition, and to follow by means of the microscope all the stages of the process. A sandstone, grit, or fine quartzose conglomerate, for example, when it approaches a mylonitic zone begins to lose its clastic or pebbly structure. The rounded grains of quartz become cracked, especially near their edges, and are then surrounded by narrow borders, consisting of detached granules: this is due to the pebbles being pressed together and forced to pass one another as the rock yields to the pressures which overcome its rigidity. Then each quartz grain breaks up into a mosaic of little angular fragments; the rounded pebbles are flattened out and become lenticular or cake-shaped. Finally only a small oval patch of fine interlocking quartz grains is left to indicate the position of the pebble, and if the matrix is quartzose this gradually blends with it and a uniform fine-grained quartzose rock results. If felspar is present it may become crushed like quartz, but often tends to recrystallize as quartz and muscovite, the minute scales of white mica being parallel to the foliation or banding of the rock, and a finely granulitic or mylonitic quartz-schist is the product. In hornblendic rocks, such as epidiorite, amphibolite and hornblende-schist, the mineral composition may remain unchanged, but very often chlorite, carbonates and biotite develop, epidote and sphene being also frequent. Biotite- and muscovite-gneisses yield very perfect mylonites, in which the micas have parallel orientation, giving the rock a flat banding and marked schistosity (see Petrology, Pl. iv., fig. 6). When these mylonitic gneisses contain pink garnet (often with kyanite or sillimanite) they pass into normal granulites; limestones, if fossiliferous, become changed into finely crystalline masses, often fissile, sometimes with lenticular or augen structure. An interesting variety of mylonite, developed in granite-porphyry and gneiss, is fine, dark and almost vitreous in appearance, consisting mainly of very minute grains of quartz and felspar and resembling flint in appearance. These form threads and vein-like streaks ramifying through the normal rocks. Examples are furnished by the flinty-crushes of west Scotland and the “trap-shotten” gneisses of south India. (J. S. F.)
MYMENSINGH, or Maimansingh, a district of British India, in the Dacca division of Eastern Bengal and Assam. It occupies a portion of the alluvial valley of the Brahmaputra east of the main channel (called the Jamuna) and north of Dacca. The administrative headquarters are at Nasirabad, sometimes called Mymensingh town. Area, 6332 sq. m. Pop. (1901), 3,915,068, showing an increase of 12·8% in the decade. The district is for the most part level and open, covered with well-cultivated fields, and intersected by numerous rivers. The Madhupur jungle is a slightly elevated tract, extending from the north of Dacca district into the heart of Mymensingh; its average height is about 60 ft. above the level of the surrounding country, and it nowhere exceeds 100 ft. The jungle contains abundance of sāl, valuable both as timber and for charcoal. The only other elevated tract in the district is on the southern border, where the Susang hills rise. They are for the most part covered with thick thorny jungle, but in parts are barren and rocky. The Jamuna forms the western boundary of Mymensingh for a course of 94 m. It is navigable for large boats throughout the year; and during the rainy season it expands in many places to 5 or 6 m. in breadth. The Brahmaputra enters Mymensingh at its north-western corner near Karaibari, and flows south-east and south till it joins the Meghna a little below Bhairab Bazar. The gradual formation of chars and bars of sand in the upper part of its course has diverted the main volume of water into the present channel of the Jamuna, which has in consequence become of much more importance than the Brahmaputra proper. The Meghna only flows for a short distance through the south-east portion of the district, the eastern and south-eastern parts of which abound in marshes. The staple crops of the country are rice, jute and oil-seeds. A branch line of the Eastern Bengal railway runs north from Dacca through Nasirabad, &c., to the Jamuna. The district was severely affected by the earthquake of the 12th of June 1897.
MYNGS, SIR CHRISTOPHER (1625–1666), British admiral, came of a Norfolk family. Pepys’ story of his humble birth is said to be erroneous. It is probable that he saw a good deal of sea-service before 1648. He first appears prominently as the captain of the “Elisabeth,” which after a sharp action brought in a Dutch convoy with two men-of-war as prizes. From 1653 to 1655 he continued to command the “Elisabeth,” high in favour with the council of state and recommended for promotion by the flag officers under whom he served. In 1655 he was appointed to the “Marston Moor,” the crew of which was on the verge of mutiny. His firm measures quelled the insubordinate spirit, and he took the vessel out to the West Indies, where he remained for some years. The Restoration government retained him in his command, and in 1664 he was made vice-admiral in Prince Rupert’s squadron. As vice-admiral of the White he flew his flag at Lowestoft in 1665, and for his share in that action received the honour of knighthood. In the following year he served under the new lord high admiral, Sandwich, as vice-admiral of the Blue. He was on detachment with Prince Rupert when the great Four Days’ Battle began, but returned to the main fleet in time to take part, and in this action he received a wound of which he died.
MYONEMES, in Infusoria and some Flagellates, the differentiated threads of ectosarc, which are contractile and doubly refractive, performing the function of muscular fibres in the Metazoa.
MYRA (mod. Dembre), an ancient town of Lycia situated a short distance inland between the rivers Myrus and Andracus. In common with that of most other Lycian towns its early history
