202 CONCHOLOGY shape, as is the case in the unattached con- chifera, the shell is said to be equivalve ; but when one valve is attached to a foreign body, this is larger and deeper than the other, and the shell is said to be inequivalve ; if one end be longer than the other, the shell is inequi- lateral. This measurement is made along the horizontal line of greatest length from each ex- tremity to the vertical line dropped from the highest point of the umbones to the base. The anterior end is usually found to be shorter than the posterior. The upper margin is called the dorsal, and the lower the ventral. The breadth is the length of the vertical line from the dorsal margin to the base, and the thick- ness is the distance between the valves when closed. The valves may join closely together, or gape open at one or both ends, or even be furnished with a vertical opening by corre- sponding notches in the margin of the valves. These peculiarities of the form of the shell have relation to the internal organization of the animal, the openings being one for the purpose of allowing the protrusion of the foot for propelling the shell, another for the byssus by which it is firmly anchored, and another for the long siphon, the respiratory organ, which serves to inhale the water to be filtered by the gills, and then to pass it back. Bivalve shells are strongly marked on their inner surface, near the dorsal margin, by the depressions for the muscles which draw the valves together, and some present prominent apophyses for the at- tachment of these adductors. The shells also bear the impressions of the attachment of the foot, byssus, siphons, and mantle. But the most important marks are the articulations of the hinge, called teeth. These, by their differ- ent numbers and positions along the dorsal margin, serve as distinguishing marks of spe- cies, and even of genera ; a distinction to which they are properly entitled from the fact that their varying forms indicate corresponding changes in the organization of the soft parts. The ligaments which hold the valves together are an external horny one lying behind the um- bones, which is stretched by the closing of the valves, and an internal short fibrous one, which is compressed endwise as the valves are drawn together by the adductor muscles. As these are relaxed by death or otherwise, the shells are pushed partly open by the elasticity of the cartilage. The marks upon the inner surface of the valves left by these organs are impor- tant features in the description and classifica- tion of bivalve shells. In the embryo of these animals the umbones are the first formed por- tions of their shelly covering. In most genera this is carried out with the growth of the body, so as always to serve as its complete protection; but in others it attains to the extent of only a partial covering, and in these instances other curious provisions are made for completing the necessary defence. The teredos are slender worm-like animals, that sometimes attain a length of 2$- ft., and one species even reaches 3 ft. By close observa- tion only are the two shells perceived at the larger extremity of this long siphonal tube. They cover the visceral organs, but ad- mit between them in front the fleshy foot, by means of which the creature is able to burrow into wood. Here he excavates long tubes adapted to his form, and finishes them with a smooth calcareous lining, thus providing an artificial covering for the unprotected portion of the body. Securing himself permanently to the inner extremity, his communication with- out is thereafter only through the siphonal tube. The pholades burrow into limestone and other stony substances which are less hard and tenacious than their own valves; other shells even are penetrated by them. The front portion of these is ribbed like a rasp, and is thought to answer the purpose of one in form- ing cavities. The whole shell is of unusual hardness, its structure more resembling that of aragonite than of calcareous spar. Being thin and without ligaments, it is strengthened by accessory valves upon the dorsal margin. In some attached bivalves, as the oyster, the em- bryonic umbo often takes the form of the sur- face to which the valve is fastened, and this form is afterward retained. With the growth of the animal the original shield loses its im- portance, and as in the univalves the spire, which corresponds to the umbo, is often de- serted, and becoming dead and brittle falls away (in which case it is said to be decollated), so in the bivalves the umbones wear out and the layers of shell scale off, leaving the summit ragged and unsightly; in this condition they are said to be decorticated. The fresh-water univalves and bivalves are especially subject to this erosion. The injury that might result from this removal of the shell is obviated in some cases by the deposit of new layers within, or by the construction of partitions, or, as in some tubular shells which continue to advance up- ward with the growth of the coral that en- closes them, by filling the deserted portion with solid calcareous matter. Some peculiarities of the shells of the other class of bivalves, the bra- chiopoda()j some recent authors placed among the articulates), are given under that head. These valves are distinguished as dorsal and ventral, instead of right and left. The ventral is commonly the larger one, and is surmounted by a prominent perforated beak, through which the organ passes by which it is attached to for- eign bodies. This valve resembles in shape the antique lamps, and the hole in the beak corre- sponds to that for the wick ; they are hence known as lamp shells. The class cephalopoda includes many genera whose shells are con- cealed within the soft parts. Many of these are known only as fossils, and nothing is left to in- dicate their former existence but the numerous stony pointed bodies and pens, which were at the same time the skeleton and partial recep- tacle of the soft portion of the animal. One of these calamary or squid-like fossils, lelemno-
