302 R. I. pooocK. 



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 coxee 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 cox^e of 

 the fourth are less masticatory in function, and the "meta- 

 stoma " 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 Palaeophonus 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 cox£e of the fourth were small and functionless so far as 

 the mouth was concerned, and food was probably crushed by 

 those of the chelse 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., PI. 19), representing in all probability the divi- 

 sional 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 



