ON THE MICROSCOPIC STRUCTURE OP SHELLS. 17 



surface (fig. 30). These appearances, however, are all reconcileable with the 

 description which I have given of this peculiar kind of structure, and are 

 easily recognized as appertaining to the group in question, and to this alone. 



4<1. When any other recent species of Terehratula is examined, an addi- 

 tional peculiarity is observed ; this consists of the presence of a large number 

 of perforations in the shell, generally passing somewhat obliquely from one 

 surface to the other, and terminating by an orifice at each (figs. 33-39). The 

 size of these perforations is sufficiently great, to enable them to be detected 

 with a hand-magnifier, as minute punctations on the surface ; and as such they 

 have been recognized by many, who have made this group their particular study. 

 I am not aware, however, that the fact of these punctations being the orifices 

 of large canals, passing from surface to surface of the shell, has been previously 

 noticed. The diameter of these perforations in the shells of recent Tere- 

 hratulcB varies from about '0006 to •0024< of an inch ; they are readily distin- 

 guished in the decalcified membrane, and are seen to be lined by a tubular 

 prolongation from it. Of their object or purpose I can give no definite ac- 

 count ; and not having had the opportunity of examining a recent specimen 

 with the animal preserved, I am unable to speak confidently as to the degree 

 of connection, which these passages have with the mantle and with the interior 

 of the shell. 



42. Having examined all the recent Terebratulce in the British Museum, 

 and in the collection of Mr. Cuming, I feel able to state as a general fact, 

 that all these species possess this remarkable character, with the exception of 

 Terehratula psittacea ; which, in the opinion of many, has other distinctive 

 characters of its own, quite sufficient to separate it from the group. Upon 

 turning my attention to the fossil species, however, a diff'erence in this respect 

 soon became obvious; for whilst some presented these perforations very 

 distinctly, others were found entirely destitute of them. The presence or 

 absence of the perforations cannot be detected in the fossil species, as in the 

 recent, by the examination of the surface of the shell with a hand-magnifier ; 

 since, owing to the filliug-up of the passages with the fossilizing material, 

 their extremities are not sufl[iciently distinguishable from the surrounding sur- 

 face. Hence, in order to determine the existence of this character in the 

 fossil species, it is necessary to make a section of the shell. Believing that it 

 must have some intimate relation with the structure and habits of the animal, 

 and that it must consequently be a character of zoological importance, I 

 have endeavoured to carry out this kind of examination to an extent suflBcient 

 to test its value ; and the following is the result of the examination of thirty- 

 five fossil species of the genus Terehratula : — 



Perforated. 

 Acuta. 

 Ampulla. 

 Bidens. 

 Biplicata. 

 Bullata. 



Caput serpentis. 

 Carnea. 

 Detruncata. 

 Digona. 

 Fimbria. 

 Globata. 

 Hemisphaerica. 

 Oblonga. 



Not Perforated. 



Coarctata. 



Concinna. 



Depressa. 



Inconstans. 



Latissima. 



Nuciformis. 



Obsoleta. 



Octoplicata. 



Plicatella. 



Reticularis. 



Rostrata. 



Spinosa. 



Subrotunda. 



1844.. c 



