PALEONTOLOGY OF NEW YORK. 



tubercled and thus could not have been adapted to the purpose of 



lation. It has been noticed in the description of the species Mesothyra 



i 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 



!in- 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 he 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 



■s;u v to accept the presumptive evidence of the existence of this plate in 



order to explain tin' mode of articulation of the valves in this species. Such 



n plate may have been simple, articulated on each edge with the valves, or 



compound, with ;i suture or hinge in the axial line, as indicated in M. Veneris. 



Although tlir 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 he horn in mind that the rostral plate, which 

 presumably tilled 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 



I.e aufficienl to destroy the membrane holding these parts in place along the 



ine 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- 



the carapace, while members of the abdomen and post-abdomen, being 



i pla.e by stronger ligaments and by the overlapping slotted edges of the 



tin- surfaces, would naturally withstand a force sufficient to separate 



the parte of the carapace. 



• preliminary Illustrations of Devonian Fossils {he. tit.) the specimens 



species is founded were referred to Dithyrocaris Neptuni, on 



similarity apparent in many features of the post-abdominal 



nore careful comparison of these parts, in the Hamilton and 



• leaves no doubt of their specific difference. This difference 



