Classification. 93 



that those of the latter are less developed in size and details. 



Mr. Matthew sospected the presence of a flap, formed by the 

 anchylosing of the appendages beneath the pygidium. From 

 the appearance of a similar structure, where the limbs are mat 

 ted together along the side of the thorax, this tentative view is 

 received with doubt. More perfect material may show distinc 

 tions not recognizable at present. 



If future investigations prove, as it now seems probable, that 

 the modified swimming joints of the endopodite are attached to 

 ten or more of the thoracic segments, the anterior eight seg 

 ments can be grouped together as the typical thorax, and the re 

 maining segments of the body as the abdomen. 



Mr. Matthew suggests that the homology between Triarthrus 

 and Limulus may not be as close as between Limulus, Calymene 

 and Ceraurus. This is true from what we now know of Triar 

 thrus, but, if a sixth pair of cephalic limbs should be discovered 

 in Triarthrus the resemblance would be strengthened. Triarthrus 

 does not differ from Ceraurus and Calymene more than would be 

 anticipated in such unlike genera. Triarthrus is essentially a 

 "Primordial" type that has continued until upper Ordovi- 

 cian time. It represents a large group of Cambrian trilobites, 

 while Calymene and Asaphus represent the more highly de 

 veloped Ordovician and Silurian forms. 



Dr. Lang held the view that if a fifth pair of cephalic limbs 

 were found, comparable to the anterior antennas "Trilobites 

 might then be regarded as original Entomostraca, to be derived 

 from the same racial form as the Phyllopoda." He says further, 

 "Xiphosura, Hemiaspidse, and Gigantostraca are themselves 

 again perhaps racially connected with the Trilobites. In any 

 case, however, in the present state of science, it seems probable 

 that all these groups are only connected at their roots with the 

 Crustacea.*" 



From the paleontological record I am essentially in accord 

 with this view, but I am not yet prepared to abandon the posi 

 tion taken in 1881, that all these groups should be arranged 

 under one class and not as an appendage to the Crustacea, as pro 

 posed by Dr. Lang. 



Text Book of Comparative Anatomy, Eng. Ed., 1891, p. 415. 

 *Loc. cit., p. 421. 



