37^ 
THE GEOLOGIST. 
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 
coriiu-vaccinum, of equal size, the outer layer of shell is an inch thick, 
whereas in the Jamaica fossil it measures 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 Ra- 
diolites. 
The upper end of the fossil was slightly convex, retaining the 
opercular valve in a somewhat damaged condition. "When split lon- 
gitudinally through the centre, it showed the body cavity, and two 
shelly processes descending from the lid (as in figure 2, a a'). Of these 
the right-hand, or posterior, apophysis {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 x). 
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 d) 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 (J) is occu- 
pied bv 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 Rippurites is accompanied by such an amount of displacement 
of the hinge as to justify the subgeneric separation of those species 
The inner layer of shells in the families Pedinida and Chamida, as well as the 
pearly lining of the Aviculidce, Turbinidce, etc., has the constitution oi Araffonite, w hile the 
outer layer consists of Calcite, as stated by Gustav Rose, and confirmed by the observa- 
tions of Mr. Sorby. The bi-axial character of mother-of-pearl may usually be detected 
with a tourmaline* in any thin, translucent section, such as a comiter or the edge of a 
pearl paper-knife. 
