134 MOLLUSCA- 
laneous shells, although they blacken speedily when put 
in the fire, and contain a good deal of animal matter. On the 
central side of this layer there is one flexible and transpa- 
rent, similar to horn, and smooth on the surface. The third ” 
layer is destitute of lustre ; and, in hardness and structure, 
resembles mother-of-pearl! shells. 
The layers which fill the concavity of this dermal plate 
are slightly convex on the central aspect, and are in part 
imbricated. Each layer is attached to the concave surface 
of the dermal plate, by the upper extremity and the two 
sides, while the inferior or caudal extremity is free. The 
inferior and first formed layers are short, occupy the base 
and middle, and rise from the plate under a more obtuse | 
angle than the new formed layers, which are both the long- 
est and the broadest. 
Each layer, which is about one-fiftieth of an inch in dia- 
meter consists of a very thin plate, the dermal surface of 
which, when viewed with a magnifier, exhibits numerous 
brain-like gyrations. From the ventral surface of this plate 
arise numerous perpendicular laminz, which, when viewed 
laterally, appear like fine parallel threads, but when examin- 
ed vertically, are found to be waved, and fold upon them- 
selves. Next the plate they are thin, and not much fold- 
ed; but towards their other extremity they become thicker, 
striated across, and more folded, with irregular margins. 
On the thick, tortuous even ends of these lamine, the suc- 
ceeding plate rests, and derives from them the peculiar 
markings of its surface. These laminz are closely set, ir- 
regularly interrupted, and occasionally anastomose. M. 
Cuvier states, erroneously, (Mém. sur la Seiche, p. 47.) 
that these laminz are hollow pillars disposed in a quincunx 
order. 
