796 The American Naturalist. [September, 
radially from the center after the manner of leaf veins, anas- 
iomosing to some degree, frequently terminating in oblique 
puncte, and in parts where the lines disappear the punctæ 
remain. 
The median plate. This is a very narrow body, its width 
exceeding at no point the greatest diameter of the rostrum. It 
begins at the union of the valves, in an acute angle, but its 
lateral margins soon become subparallel or convex outwardly. 
In relative proportions its length is about eight times its width. 
Along the median axis it bears an elevated ridge-like line, 
from which there is a gentle slope on each side, and it was 
essentially from this evidence of continuity of the test on the 
axialline and from the concealment of the sutures between 
this plate and the valves, that the carapace was originally 
described as univalvular. The fine incised lines of the surface 
diverge anteriorly from this ridge. 
In discussing, in Volume VII of the Paleontology of New 
York; the structure of the great carapaces from the lower 
Fic. 8. Mesothyra oceani. 
Chemung beds at Ithaca, N. Y., which had theretofore beer: 
known as Dithyrocaris neptuni, it was shown that these cara- 
paces (now termed Mesothyra oceani) must have had a compo- 
sition similar to that we now find to have actually existed in 
3 Op. cit. p. 184, pl. xxxi, figs. 8-10. 
