190 
PALEONTOLOGY OF NEW YORK. 
strongly tubercled and thus could not have been adapted to the purpose of 
articulation. It has been noticed in the description of the species Mesothyra 
Veneris that a short distance within the dorsal margin of the type specimen, and 
parallel to it, is an impressed line which merges into the hinge at a point 
corresponding in position to the angular process in this species. If this groove 
represents a suture or a line of symphysis, and the narrow strip of carapace 
between it and the dorsal line be removed, the outline of the dorsal margin of 
this species would be similar to that in M. Oceani. In two of the specimens at 
hand there is evidence, though not conclusive, of such a median strip in this 
species, detached from, and lying alongside the dorsal margin. It seems 
necessary to accept the presumptive evidence of the existence of this plate in 
order to explain the mode of articulation of the valves in this species. Such 
a plate may have been simple, articulated on each edge with the valves, or 
compound, with a suture or hinge in the axial line, as indicated in M. Veneris. 
Although the existence of this plate has not yet been positively demonstrated, 
nor the necessity of its existence recognized in species heretofore referred to 
the genus Dithyrocaris, it must be born in mind that the rostral plate, which 
presumably filled the anterior cleft between the valves, so far as known, has 
not been seen in any species of the genus. 
It will be readily conceived that a very slight degree of maceration would 
be sufficient to destroy the membrane holding these parts in place along the 
line of symphysis, and bodies so light as the rostrum and such a median plate, 
would be readily transported, even in quiet water, away from the heavier por¬ 
tions of the carapace, while members of the abdomen and post-abdomen, being 
held in place by stronger ligaments and by the overlapping slotted edges of the 
articulating surfaces, would naturally withstand a force sufficient to separate 
the parts of the carapace. 
In the preliminary Illustrations of Devonian Fossils (loc. cit.) the specimens 
upon which this species is founded were referred to Dithyrocaris Neptmi, on 
account of the similarity apparent in many features of the post-abdominal 
parts, but a more careful comparison of these parts, in the Hamilton and 
Portage specimens leaves no doubt of their specific difference. This difference 
