42 NEW YORK STATE MUSEUM 



Limulus is demonstrated; it is shown that the chelicerae of 

 Pterygotus consist of but three joints as those of the other 

 eurypterids, the long proximal joint being undivided. 



The probable mode of life of these strange creatures has been 

 subjected to a thorough discussion and the conclusion reached 

 that their life habits were not always uniform; on this basis 

 they can be divided into four groups, as evinced by the form of 

 their bodies and appendages ; these groups being typically 

 represented by Pterygotus, Eusarcus, Eurypterus and Stylonu- 

 rus. In the first, forms of principally the swimming habit pre- 

 vailed; the second were mud-grovelers and crawlers; the third 

 are slightly specialized forms that were crawlers, burrowers and 

 swimmers ; the fourth were crawlers. 



From the character of the rocks in which the Eurypterida 

 occur and from the associate faunas, it is inferred that the euryp- 

 terids were originally marine animals, but. at the time of their 

 climacteric development in the Upper Siluric had become in- 

 habitants of the lagoons and estuaries and were typically eury- 

 haline, i. e. able to live in both salt and brackish water, and that 

 this habit which they held throughout the Devonic, led them finally 

 into the evident fresh-water habit of the expiring species in the 

 coal lagoons of the Carbonic and Permic. 



The collection of larval stages (some but i millimeter long) 

 of the genera Eurypterus, Eusarcus, Stylonurus, Hughmilleria 

 and Pterygotus has furnished data on the ontogeny of these 

 genera which allow the following general inferences: (i) that 

 the carapace is relatively larger in the larval stages, (2) that 

 the compound and lateral eyes are relatively much larger than 

 in the mature stage; (3) that they are nearer the margin; (4) 

 the ocellar mound is large and more prominent; (5) the swim- 

 ming feet are larger; (6) the abdomen lacks the distinct differ- 

 entiation into pre- and postabdomen; (7) the number of seg- 

 ments is less; and (8) the telson spine appears to have been less 

 developed. 



Of these characters the relatively larger size of the carapace, 

 compound eyes and swimming feet, and the smaller number of 

 the abdominal segments are considered as cenogenetic or purely 

 larval characters, while the approximation of the compound eyes 

 to the margin, the prominence of the ocelli and their tume- 

 scences, the lack of differentiation of the abdomen and the smaller 

 size of the telson, are held to be palingenetic and of phylogenetic 



