436 PHILOSOPHY OF ZOOLOGY. 



According to our observations, the dermal plate appears 

 to consist of three different laminae, arranged parallel to 

 one another. The external or dorsal layer is rough on the 

 surface, and marked by obscure, concentric arches towards 

 the summit, formed by minute knobs, which become larger 

 towards the base, where they appear in the form of interrupt- 

 ed transverse ridges. It is uniform in its structure, and the 

 tubercles possess a poli sh and hardness equal to porcellaneous 

 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 struc- 

 ture, 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 ob- 

 tuse angle than the new formed layers, which are both the 

 longest 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 

 examined vertically ? are found to be waved, and fold upon 

 themselves. Next the plate they are thin, and not much 

 folded ; but towards their other extremity they become 

 thicker, striated across, and more folded, with irregular 



