lx 
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 accessory evidence, would naturally lead to their allocation to the same 
zoological position. The earliest of these genera established, viz., Pelto- 
c.aris, 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 
aftinities appear the more decided; the anterior cleft for the rostral plate, the 
posterior cleft for the protrusion of the abdomen, and the sides normally 
sloping as in Peltocaris, Rhinocaris, etc. 
On plate 35 of this volume is given a figure representing a minute bipennate 
body having the outline of Dipterocaris, 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 shales of the Portage group, in which Goniatites complanatus is the most 
