134 MOLLUSC A. 



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 laminae, 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 lamina?, the suc- 

 ceeding plate rests, and derives from them the peculiar 

 markings of its surface. These laminae are closely set, ir- 

 regularly interrupted, and occasionally anastomose. M. 

 Cuvier states, erroneously, (Mem. sur la Seiche, p. 47.) 

 that these laminae are hollow pillars disposed in a quincunx 

 order. 



