ON THE MICROSCOPIC STRUCTURE OF SHELLS. 15 



the life of the animal, it is impossible now to say ; as there is no existing 

 wroup, to which the Rudistes seem to bear any close resemblance. The shape 

 of each is usually that of a very short hexagonal prism, terminated at each 

 end by a flat partition : consequently a section in one direction wijl exhibit 

 the walls of the chambers disposed in a hexagonal network (fig.j2^) ; whilst, i, *:> 

 ■when the section passes in the opposite direction, the transverse partitions 

 come into view (fig. ^). The cancellated structure is externally and inter- ^ . 

 nally covered with a shelly plate, in which no perforations whatever can be 

 seen. It is difficult to imagine, therefore, how any communication could 

 have existed between the animal contained within the shell, and the cancel- 

 lated structure which forms its thickness. 



33. The only approaches to this structure, so far as I am aware, presented 

 by any recent shells, are to be found in the irregular cancellated structure of 

 the base of some of the sessile Cirrhopods ; and iu a similarly irregular can- 

 cellated structure, which has been described by Mr. Gray* as existing be- 

 tween the laminae of an undescribed species of Oyster, named by him Ostrea 

 purpurea, I have not myself met with anything at all to be compared with 

 it among the shells of ordinary MoUusca ; and I cannot but think that its ex- 

 istence, as nearly the sole component of their shells, marks out the Rudistes 

 as a group altogether distinct from them. The position which I should be 

 myself inclined to assign to them, from the structure of the shell, is between 

 the OstracecB and the sessile Balani ; and I believe that the most complete 

 information we possess on the character of the animals, would lead to the 

 same conclusion. 



34. The presence of this structure in any fossil, whose situation is doubt- 

 ful, appears to me a sufficient reason for referring it to the group of Rudistes, 

 Thus from finding it in Pleurorhynchus Hibernicus (figs. 24, 25), 1 should 

 almost unhesitatingly assign this position to that shell, notwithstanding its 

 strong resemblance in form to some of the Cardiacece. It has not the least 

 correspondence, however, to the Cardium cardissa, or to any of the Cardiacete 

 that I have examined, in regard to the structure of its shell, which entirely 

 consists of cancellated texture, — the cancelli being formed by the intersec- 

 tion of planes at right angles to each other. When the shell disintegrates, 

 the casts of these cancelli, which are produced by the infiltration of carbonate 

 of lime, are disposed to separate from each other ; and thus a layer of iso- 

 lated parallelopipeds are found in place of the shell. 



35. Having now described the principal component elements, of Avhich the 

 shells of MoUusca are made up, I proceed to detail the results of my inquiries 

 into the combination of these, in the several groups Avhich altogether form 

 this sub-kingdom. From what has been already stated, the question natu- 

 rally presents itself, how far the elementary structure of the shell may furnish 

 characters of importance in classification and in the determination of fossils. 

 My inquiries, so far as they have yet proceeded, tend to establish this po- 

 sition, that ivhere a recognizable and constant diversity presents itself in the ele- 

 mentary structure of the shell among different groups, that diversity affords 

 characters, ivhich are to a very high degree indicative of the natural affinities 

 of those groups. It is not always that peculiarities sufficiently distinctive pre- 

 sent themselves, even between what are regarded zoologically as distinct fami- 

 lies ; but where a marked diversity does exist, I believe that it will always be 

 indicative of the affinities of the animal. Thus the conformity in structure 

 between all the shells of one natural family is usually so close, that any 

 strongly-marked difference in a particular genus would make me hesitate in 



* Loc, cif. 



