CHAMACEA. 



733 



ous transverse lamellae, which are horizontal, or are directed somewhat 

 obliquely outwards and upwards, the intersection of the two series giving 

 rise to a lattice-like structure (fig. 614). The shell splits easily along the 

 line of these transverse lamellae, the upper surfaces of which — as also of 

 the thickened upper margin of the shell — exhibit radially disposed vas- 

 cular impressions. The inner stratum of the shell is white, porcellanous 

 and laminated in structure. Sometimes the parallel lamellae are sepa- 

 rated by vacant spaces (fig. 613), thus giving rise to intervening lacunae 

 (' water-chambers '). This is especially the case in Hippurites, in which 

 the greater portion of the conical lower valve is formed by the inner 

 shell-layer. The two shell-layers separate readily from one another ; and 



Fig. 612. — Hippurites Toucasi- 

 anus. A large individual, with two 

 smaller ones attached to it. Creta- 

 ceous. 



Fig. 613.— Vertical section of a 

 broken shell of Hippurites organ- 

 isaus, without the body-chamber, 

 showing the horizontal partitions 

 and intervening spaces ("water- 

 chambers"), of the natural size. 

 (After Zittel.) 



as the inner one, at least in Sphcerulites and Radiolites, resists destruc- 

 tion during fossilisation less completely than the outer one, it commonly 

 happens that only the outer layer is preserved, and the mould of the 

 body-chamber appears to be separated from the latter by a vacant space. 

 Still more commonly, the inner layer has undergone transformation, and 

 has been replaced by crystalline calcite. The upper valve consists, like 

 the lower one, of two layers, but the cellulo-prismatic outer stratum is 

 mostly of small thickness, and in Hippurites is traversed by a compli- 

 cated system of canals, while the inner porcellanous and laminated layer 

 is frequently converted into crystalline calcite." It is usually supposed 

 that the outer layer of the shell in the Hippuritidce corresponds with the 

 external prismatic layer in such Bivalves as Pinna; but it differs from 

 the latter in the great size of the component prisms, and in the fact that 



