.37^ 



THE GEOLOaiST. 



are always visible in European specimens of the hippiirite, which 

 have become hollow by the dissolution of their inner layer of shell,* 

 (fig. 7). These ridges are formed by the folding in of the outer 

 wall of the shell, and it is evident that the numerous rays of the 

 Jamaica fossil are produced by a repetition of the same process. 

 They seem intended to compensate the tenuity of the outer wall, and 

 perhaps are the cause of its reduction. In a specimen of Hippurites 

 cornu-vaccinum, of equal size, the outer layer of shell is an inch thick, 

 whereas in the Jamaica fossil it m.easures only three lines, and in a 

 transverse section (fig. 4) exhibiting the lateral union of three 

 (probably small) individuals, the double boundary-wall is less than a 

 line in thickness. In the sections represented (figs, 2, 5), the outer 

 shell-wall has chiefly been removed by accident or destroyed by mining 

 parasites, except where preserved by investing corals and small 

 diolites. 



The upper end of the fossil was slightly convex, retaining the 

 opercular valve in a somewhat damaged condition. AVhen split lon- 

 gitudinally through the centre, it showed the body cavit}^, and two 

 shelly processes descending from the lid (as in figure 2, a a'). Of these 

 the right-hand, or posterior, apopliysis (a) projects into a cavity, which 

 is so close to the principal inflection (m) that part of it is shown in 

 the same figure. The beads of the rays in the transverse section 

 are strung together by almost invisible lines ; but in this longitudinal 

 fracture they are seen to be continuous plates, and are striated on 

 the side by lines of growth. At the summit they must have formed 

 a series of radiating ridges, with furrows between, bordering the in- 

 terior of the valve. The bottom of the body-cavity was also more 

 irregular than usual in shells. The upper valve is perforated by a 

 few large radiating canals, with canaliculi conducting to the outer 

 surface (x jt). 



After it came into my possession, a fresh section was made across 

 that half of the cylinder which contained the dental apparatus, in 

 order to show the exact form and position of the hinge-teeth. They 

 are seen in the figure (5, a a) filling their sockets exactly, with the 

 exception of small defined spaces on their outer sides, which form 

 the only trace, at this level, of the cavities occupied by the divided 

 cartilage {cc). The interval between the dental sockets (I) is occu- 

 pied by a solid, rectangular portion of shell, representing the single 

 dental process of the lower valve. There is no " ligamental inflection " 

 of the outer shell, as in H. cornu-vaccinum, and many other species. 



The existence of the ligamental plate in the typical division of the 

 genus Hippurites is accompanied by such an amount of displacement 

 of the hinge as to justify the subgenerie separation of those species 



The inner layer of shells in the families Fecthndie and CJiamida, as well as the 

 pearly lining of the Avic'C-il,,--^ Ti'rbiniihr, etc., has the constitution oi Aragonite, while the 

 outer layer consists of Calcili', as slated l)y Gustav Rose, and confirmed by the observa- 

 tions of jNIr. Sorby. 'The hi-axial character of mother-of-pearl may usually be detected 

 with a tourmaline in any thin, translucent section, such as a counter or the edge of a 

 pearl pa[)cr-knife. 



