178 ON THE ANATOMY AND AFFINITIES OF 
figured in a state of greater or less perfection in Plate II]. [Plate 14] 
figs. 2, 7. 
The most perfect specimens (figs. 2, 5,) reveal a broad plate 
having one edge nearly straight, but presenting a slight median 
emargination, while the opposite edge inclines from the middle line 
on each side outwards towards the straight margin, which it would, if 
continued, cut at a considerable angle. 
In the middle line a slight longitudinal depression passes along 
the face of the plate for a short distance, from the straight margin 
towards the convex one, and then divides into two diverging 
grooves. 
From the extremities of these, two others, slightly convex 
towards one another and the median line, are continued as far 
as the convex edge, and thus the plate becomes divided into three 
regions,—a narrow “ median lobe” and two wide “ lateral ale 
The inner angles of the ale are rounded off; the median lobe 
projects beyond them and ends in a sharply defined, well-rounded, 
free edge. The outer fourth of the transverse diameter of the 
median lobe on each side is overlapped by the corresponding ala, a 
considerable amount of matrix being occasionally interposed between 
the two. The general plane of the median lobe, in fact, lies behind 
that of the ale, whence it arises that impressions of the plate 
present the aspect figured by M. Agassiz, and reproduced in 
Plate III. [Plate 14] fig. 2, and the median lobe, while covered 
by the ale, appears spoon-shaped, whereas its end is in reality but 
little expanded (fig. 7). 
The ale present, very well developed, the peculiar sculpture so 
characteristic of Pterygotus and its allies, multitudes of little semi- 
lunar facets, with their convex edges raised and all turned towards 
the convex margins of the plate, being thickly scattered over its 
surface. 
The facets of the sculpture on the middle of the median lobe are 
greatly elongated, but have the same general direction. The rounded 
end of the median lobe is marked by radiating striae, while its sides, 
where overlapped by the ale, are smooth. From the thinness and 
sharp definition of its edge, the median process would seem to 
have been quite unconnected with the ala laterally ; and it appears 
to have very readily yielded to force along the diverging grooves. 
The impressions of median lobes detached, in consequence of having 
broken away from the ala along these lines, are figured in Plate III. 
[Plate 14] figs. 3, 4. 
No crustacean, living or extinct, is known to have a carapace 
” 
