Ix SYNOPSIS OF GENERA. 



function to the aptychi of the Ammonites. All the representatives of the 

 genera mentioned show a degree of analogy in form and structure which, with- 

 out accessor}' evidence, would naturally lead to their allocation to the same 

 zoological position. The earliest of these genera established, viz., Pelto- 

 caris, Discinocaris and Aptychopsis are of undoubted crustacean nature, as they 

 show evidence of a cephalic or rostral plate filling the anterior cleft, and 

 moreover, they are found at Silurian horizons where Goniatites are unknown. 

 Referring to the Devonian species, it must be taken into consideration that 

 they occur at horizons which are usually prolific in Goniatites, but while some 

 may be cephalopodous, as the two species mentioned, those described by Key- 

 serling from the Domanik-schiefer of Petschora-land, by Woodward from the 

 black shales of Biidesheim, and those from the State of New York occur in no 

 intimate or suggestive association with these fossils. The species Spathiocaris 

 Emersoni has proven an abundant fossil at certain localities in the lower shales 

 of the Portage group, but it has not as yet been seen in intimate connection 

 with Goniatites. On the other hand it is found at an horizon in which the 

 Phyllocarid Crustacea appear to have attained their maximum development in 

 this country. 



Of the bipennate species, or those with the univalvular shield cleft at both 

 anterior and posterior extremities (Dipterocaris, Pterocaris), we have no means 

 of positively determining the nature, either from analogy with the aptychi of 

 cephalopods or from similarity to any undoubted crustacean, except as shown 

 in the genus Peltocaris, etc. With our present knowledge their crustacean 

 affinities appear the more decided; the anterior cleft for the rostral plate, the 

 posterior cleft for the protrusion of the abdomen, and the sides normally 

 doping as in Peltocaris, Rhinocaris, etc. 



On plate 35 of this volume is given a figure representing a minute bipennate 

 body having the outline of JJipterocaris, lying within the body-whorl of a large 

 individual of Goniatites complanatus. The body-whorl of the shell has a width 

 of 37 mm., the diameter of the entire shell having been as great as 75 mm.; 

 the length of the Dipterocaris-like body is 5 mm. The specimen is from the 

 lower Bhales of the Portage group, in which Goniatites complanatus is the most 



