460 GEOLOGY OF OHIO. 



thoracic members without them, they could not be other than antennal 

 in their functions, notwithstanding their great size and anomalous char- 

 acter. Taking this view of their nature, the specimen would conform 

 strictly to the type of Macrouran Decapods. 



In its generic relations, as well as in its general expression, the speci- 

 men resembles most nearly the genus Pygocephalns of Prof. Huxley, 

 first given in the Quart. Jour. Geol. Soc. London, vol. 13, p. 363, 1857, 

 with figures and descriptions of three specimens, under the name P. 

 Cooperi. Neither the genus or the species were well characterized at 

 that time. It is however again referred to in vol. 18, p. 420, of the same 

 journal, with a figure of a specimen supposed to be of the same species, 

 much better preserved, from the coal shales at Paisley. There are how- 

 ever, too many limbs represented as originating from the thorax for a 

 Decapod, and the antennae, although represented as of large size are not 

 like those of the Ohio specimen, while there is a second pair shown. In 

 other parts the figure is indistinct, and in the description the parts are 

 not defined sufficiently for close comparison. The differences, however, 

 are so great that I shall propose for this form the new generic name 

 Pal^opal^Smon, with the following diagnosis : 



Genus PALiEOPAD^BMON Whitfield. 



Am. Jour. Arts and Sci., 3d series, vol. 19, p. 40, 1880. 



A marouran decapod crustacean, having a shrimp-like body, with a thoracic 

 carapace narrowed but not rostrate in front, and keeled on the back and sides. 

 Abdomen of six segments terminated by an elongated triangular and pointed telson ; 

 segments arched, pleura smooth, not expanded nor lobed, their extremities rounded. 

 Sixth segment bearing caudal flaps, one on each side, composed of five visible ele- 

 ments, the outer four apparently anchylosed to form a single large triangular plate 

 on each side of the telson. Thoracic ambulatory appendages elongated, smooth, 

 and filiform, except the upper (second) joint, which is laterally compressed. Abdom- 

 inal appendages short, the upper joints flattened or convex anteriorly, as if for the 

 attachment of plates or fimbriae. Antennae with the basal joints strong and well de- 

 veloped, of large size, much exceeding in strength any of the thoracic limbs. Eye 

 pedunc'es short. 



This is so far as I am aware the most ancient decapod crustacean 

 3-et recognized, and on that account alone is of great interest. The 

 character of the caudal plates, in having the parts combined to form a 

 solid plate on each side of the telson, is also an interesting feature, if 

 rightly understood. From the impression of the plate as seen on the 

 ventral side, it was at first supposed to be a simple element only, but on 

 obtaining an impression in the fragment of rock, chipped from the top 

 or dorsal surface, the obscure lines of the first and second joints were 

 detected, while the outer three are only traceable from the very slight 

 difference in the surface character of two of them, and the thickened 

 substance of the third or marginal one. Of the thoracic limbs only parts 

 have been seen, and of the abdominal members the three anterior ones 

 on one side; the others being concealed by the rock. The e5 r e-stalks ap- 



