INTIMATE STRUCTURE OF THE SHELLS OF BRACHIOPODA. 37 



laminge of the shell around them. Where the shell is furnished with spines, as is 

 especially the case with Prodtictus horridus, the perforations are continued into them ; 

 and such passages are of more than the average dimensions. The shells of this group 

 have for the most part undergone such a metamorphic action, that I cannot speak 

 confidently with regard to the conformity of their texture to that of the Brachiopoda 

 generally ; but for the reasons already given (p. 24), I have no reason to beheve it to have 

 been different. I am disposed to thmk, however, that in some of the massive shells of 

 this group, which appear to have been formed by the addition of successive laminae to the 

 interior, the internal laminae were sometimes destitute of perforations — at least at a 

 distance from the margin — as stated by Mr, D. Sharpe. Still, unless this point can be 

 satisfactorily made out in shells whose original texture have been well preserved (if such 

 should fortunately present themselves), it would be unsafe to make a positive affirmation 

 regarding it. 



Productus. — Of this genus I have examined the following species; — Cora, ¥leniingii, 

 horridus, and striatus, all of which have well-marked perforations. 



Strophalosia. — The two species of this genus which I have examined, viz., 

 Sedgwichii and sub-aculeata, are both perforated like Productus. 



Chonetes. — Of this genus, also, I have examined two species, armata and lata, both 

 of which are perforated. 



Genus — Calceola. 



Though I have examined several specimens of Calceola Sandalina, I have not met 

 with one in which the texture was sufficiently well preserved to enable me to speak 

 positively as to its character. I have no reason, however, to regard it as having 

 departed from the ordinary type ; and I have not been able to detect any distinct traces 

 of perforations. 



Genus — Crania. 



The structure of the shell in this genus, as determined by the examination of the 

 recent Crania norvegica, is widely different from that of Brachiopoda generally. Instead 

 of a series of flattened prisms arranged with great uniformity, we only meet with a 

 substance which does not present any regularity or distinctness in the arrangement of 

 its comp inents, but which is not at all unlike that of which many Lamellibranchiate shells 

 are composed, and may probably, like it, be regarded as having been originally formed 

 by the coalescence of cells, which were destitute of any constancy in size, shape, or 

 general arrangement. But whilst departing from the general Brachiopodous type in this 

 respect, the shell of Crania is quite conformable to it, in being penetrated by canals 

 which are prolonged from the fining membrane of the shell, and which pass towards its 



