'230 On a peculiar Structure in Shells, 



filled up the cavity between them, though it seems probable that 

 they may have arisen from some peculiarity in the mantle of the 

 animal, developed only when the new laminae were about to be de- 

 posited, and not present or shrunk when the smooth upper surface 

 of the lamina was formed, for it is evident, from the nature of the 

 surface of some specimens, that the parietes of the cells are very 

 gradually deposited on the smooth upper surfaces of the transverse 

 plates of growth. The vein-like grooves above described do not 

 seem to exert any influence over their form, for they are apparent- 

 ly not in any way connected with the distribution of their parietes, 

 while yet they show that there must exist some peculiarity of the 

 mantle to form such peculiar grooves. 



These shells, and the Hippurites, have occupied considerable at- 

 tention of late, on account of the difficulties which arise in deter- 

 mining their place in the animal kingdom ; for although evidently 

 bivalvular, yet they differ in several particulars from both the free 

 bivalve shells of the Conchifera and the lamplike bivalves of the 

 Brachiopodes, not having the ligaments nor the apical umbones of 

 the former, nor the numerous muscular scars so characteristic of the 

 Crania, which alone resemble them in form among the latter. Two 

 French authors have attempted to explain this difficulty. M. De- 

 france and others having observed that the cast on which the genus 

 Birostrites has been formed is always found in the cavity of these 

 shells, and that as there is a space between the cast and the parietes 

 of the shell, M. Deshayes concludes that the Sphaerulites are con- 

 chifera provided with a toothed hinge and ligament, and allied to 

 the genus Spondylus, the inner coat of which is lost in the act of 

 fossilization. M. Desmoulins on the other hand believes them to be 

 the shelly cases of a new class of animals of which he ventures to 

 give a theoretical description, allying them to the Ascidia, believing 

 the space between the cast and the shell to be filled up with the 

 cartilaginous mantle of the mollusc. Unfortunately none of the 

 specimens, either from the chalk or the limestone strata, that have 

 come under my notice, exhibit the internal cast as here described, 

 but the specimens from the chalk certainly throw a doubt over both 

 theories, for some have one or more oysters attached to the inner 

 surface of their cavity, and others are pierced with minute branch- 

 ed worm marks exactly like the worm marks so common on the sur- 

 face of existing shells. These facts prove that whatever may have 

 been the structure of the substance which filled up the space said 

 to have been lost in fossilization, (if any such substance ever was 



