cepting those of the first pair, acted as jaws, and were frequently armed with teeth, the greatest share in crushing and masticating food falling to the coxæ of the fourth pair, which were especially enlarged for the purpose. Behind, and partially concealing them from the ventral side, lay a large plate, the so-called "metastoma," the homologue of the scorpion's sternum. To all intents and purposes the same arrangement is found in Limulus, except that the coxæ of the fourth are less masticatory in function, and the "metastoma" is represented by a pair of moveable sclerites, the "chilaria," set immediately behind and between the bases of the legs of the fourth pair.
In Palæophonus the sternal plate of the prosoma lies apparently behind the basal segments of the fourth legs as in Limulus, and, as in the latter and in the Eurypterida, the basal segments of all the appendages were in contact or capable of meeting in the middle line. On the other hand, the coxæ of the fourth were small and functionless so far as the mouth was concerned, and food was probably crushed by those of the chelæ as in recent scorpions, the sterno-coxal sclerites of the second and third pairs assisting in this process, and preventing the escape of nutritive juices. Thus, so far as the parts now under discussion are concerned, this archaic scorpion presents a condition of things intermediate in many particulars between that of the typical scorpions and of Limulus or Eurypterus.
Mesosoma.—The ventral portion of the first somite of the mesosoma is represented by a relatively short but wide area lying behind the sternal region of the prosoma. This area is marked in the middle line with a short longitudinal groove (gen., Pl. 19), representing in all probability the divisional line between the right and left halves of the genital operculum. On each side this area is impressed with a shallow but conspicuous indentation, which from its position seems hollowed out for the reception of the third segment of the fourth leg, perhaps in order that this portion of the appendage might be insunk to the level of the generative
