their Secretions, 



613 



breadth; slightly convex on the back, and considerably pro- 

 tuberant on the anterior side, where it is so soft that the nail, 

 applied with a moderate force, cuts it easily. The figure a 

 (Jig, 105.) is a front view of this production, reduced to nearly 



two ninths of the natural size : the convexity is greatesV in the 

 middle, whence it gradually declines towards both ends ; the 

 upper half being smooth and obtuse, while the lower is marked 

 with imbricated curved lines, and contracts rather suddenly 

 to a narrow point. But this point is surrounded with a broad 

 prominent concave border, consisting of two portions closely 

 united : the first opaque, and scored longitudinally with fine 

 lines; the other perlaceous, and faintly lined transversely. 

 The latter is continued not only along the margins, but ex- 

 tends over the whole opposite surface (b), which, from this 

 circumstance, is much harder than the anterior; and it is 

 also all over roughened with slightly raised knobs or granula- 

 tions, arranged in obscure curved lines. To complete the 

 view, I give two sections of the bone {c, a perpendicular, 

 and d, a transverse, section), which will likewise disclose 

 in part the internal structure. You perceive that the bellied 

 portion is composed of very thin parallel and convex plates, 

 which run slopingly from the anterior to the posterior sur- 

 face ; and that the intermediate spaces are filled with a much 

 less compact material, formed of fibres in a crystalline state, 

 placed perpendicularly, as is more clearly exhibited in the 

 following figures [e and^). The plates are of unequal lengths; 

 the lowermost, which are also the first formed, being the 

 shortest, while those of the latest formation are the longest ; 

 and yet, however, they cover only half of the whole sur- 

 face. The fibres of the intermediate substance are loosely 

 connected together by other fibres running in the direction of 

 the longitudinal plates, as is very obvious when a small 

 and thin section is viewed through a magnifier (f)v the 



R R 3 



