WATERLIME GROUP. 401* 



From the preceding descriptions and illustrations, the relations of all these parts 

 will be easily understood. 



- The Genus Eurypterus differs from Ptervgotus in having, according to the 

 figure of P. hilohus of Salter, one more articulation in the body; and while the 

 caudal articulation of that species is bilobed, that of all the Eurypteri, so far as 

 known, is simple, elongate and spiniform. The anterior feet of the Eurypterus have 

 the joints furnished with smooth chelate tips, which are not prehensile as in 

 Pterygolus : the articulation of the natatory feet in the two genera is likewise 

 distiiiclive (as shown in the figure of Mr. Salter). The eyes of Eurypterus are 

 usually, or perhaps always, within the margin of the carapace, while those of 

 Pterygotus are marginal. The central oculiform tubercles shown in the carapace of 

 Eurypterus have not been shoAvn in Pterygotus. 



In the scaly covering of the body, I have not been able to discover any impor- 

 tant differences between that of Eurypterus and that of other specimens which I 

 have reason to believe are Pterygotus. There is likewise great apparent similarity 

 of structure in other parts of these peculiar crustaceans. 



The cordiform " scale-like appendages", one of which is figured by Mr. Salter*, 

 appear to me to be tiie epistoma, or plate covering the bases of the first joints of 

 the posterior pair of feet, and lying just behind the mouth ; and though no such 

 organ has yet been shown as belonging to Pterygotus, they may have had similar 

 relations to that fossil. 



The palette in the extremities of the natatory feet of Pterygotus is very similar 

 to the corresponding part of Eurypterus, though much more developed j and the 

 number of joints in these organs do not correspond in the two genera. 



This terminal palette has never been noticed, so far as I am aware, until the 

 present time; and in the two previously known American species of Eurypterus it 

 is extremely small, while in one species it equals the preceding joint in size. 



The number of joints in the feet is much greater in Eurypterus than in Pterygotus, 

 according to Mr. Salter's figure. The number of joints of the body, as shown by 

 the same figure, would correspond with those of Eurypterus, admitting the first one 

 on the lower side to consist of a double articulation soldered together, and repre- 

 senting two joints of the dorsal side. 



The bilobed character of the last articulation is a distinguishing .feature, and 

 doubtless of generic importance; and it is not probable that any species with spini- 

 form tails will be found to be true Pterygotus. 



• Quarterly Journal of the Geological Society, Vol. xv, pag. 28, flg. 7. 

 f Pal^sontoloot III. J 51 • 



