CRUSTACEA. 191 



consists of the following points : in M. Oceani, (a) the caudal plate is smaller, 

 less robust, and is covered with low tubercles ; (b) the telson is relatively much 

 longer, though not as long as the cercopods (this feature is incorrectly repre- 

 sented on plate xxxiii, fig 7, where the telson is given about the same relative 

 length as in M. Neptuni; at least a half-inch should be added to the length of 

 the figure of this spine) ; (c) the carina on the telson is much narrower and 

 more sharply angled; (d) both telson and cercopods are conspicuously tubercled, 

 and (e) the linear ornamentation is less marked. 



Distribution. This elegant species is represented by portions of four cara- 

 paces, four abdomens and post-abdomens and two post-abdomens without 

 somites attached. All are from a single locality in the Portage group. In 

 the sandy shales bearing Plumalina Vanuxemi, Dawson : Ithaca, Tompkins 

 county. 



Mesothyra Neptuni. 



PLATU XXXII, FIG. 7; and PLATE XXXIII, FIG. 1. 



Dithyrocaris Neptuni, Hall. Sixteenth Rept. N. Y. State Cab. Nat. Hist., p. 75, pi. i, &g. C .K 1863. 



Diihyroearis Neptuni, Hall. Illustrations of Devonian Fossils, pi. xxiii, fig-. 6. 1876 ; not pi. xxii, tigs. 1-5 ; 



pi. xxiii, figs. 1-3. 

 Dithyrocarte Neptuni, Packard. Monog. N. Amer. Phyllopod Crustacea. Twelfth Ann. Rept. U. S. Geol. 



Surv., p. 452, fig. 73. 1883. 

 Dithyrocaris Neptuni, Ethkridgk, Woodwakd and Jones. Fifth Rept. Committee on Fossil Phyllopoda of 



the Palseozoic Rocks. 1S87. 



Cephalothorax and abdomen unknown. 



Post-abdomen. Caudal plate sub-quadrangular in outline on the dorsal side, 

 strongly convex, and flattened above; produced into a broad, stout, relatively 

 short telson. Along the axial line of this spine is a broad, low ridge, 

 which runs to its apex and is most strongly elevated just behind the insertion 

 of the spine upon the caudal plate. The cercopods are long, nearly one-half 

 longer than the telson, flat and with a low ridge on their inner edges. To 

 these ridges were attached the setaceous fimbriae which are faintly visible 

 on one of the lateral spines of the type specimen. The articulating edge of 

 each cercopod is curved upward and forward into a smooth marginal ridge, 

 which is overlapped by the adjacent edge of the caudal plate. 



The surface is marked by strongly elevated, concentric, sometimes anas- 



