386 Dr. W. B. Carpenter on the Minute Structure of 



of Pandora, to which I have already adverted, I have not met with 

 a single exception to the rule, — that marked differences in the 

 structure of shell go along with marked differences in general 

 characters, and that a close correspondence in the structure of 

 the shell may be held to indicate a tolerably close natural affinity. 

 Many circumstances which at first appeared to me exceptional, 

 were explained in the course of my investigations, by my finding 

 (chiefly from the notes of M. Deshayes to the new edition of 

 Lamarck) that the views of systematists had recently undergone 

 modification, on several points, from increased knowledge of the 

 structure of the animals, so as, in fact, to become precisely accor- 

 dant with the inferences which I should have been disposed to draw 

 from the structure of the shell alone. 



In concluding this part of my subject, it may be interesting to 

 my geological auditors, if I say something in regard to the effects 

 oi fossilization under different circumstances, on the structure of 

 shell. The fossil shells I have examined have been chiefly from 

 the carboniferous or mountain-limestone, lias, and oolite. Those 

 from the carboniferous limestone are frequently changed by a 

 process of crystallization, to such a degree that no organic struc- 

 ture is discernible in them ; but the characteristic structure of 

 the Brachiopoda is usually well preserved. The membranous 

 shells of the lias are usually well adapted for microscopic in- 

 quiry ; but they split with unusual facility into layers, as if they 

 had undergone very prolonged maceration. The cellular shells, 

 from the same cause, are disposed to disintegrate into their com- 

 ponent prisms, so that it is difficult to prepare them for the mi- 

 croscope. On the other hand, the membranous shells of the 

 oolite are apt to be entirely changed by crystallization, so that 

 not a trace of the organic structure is preserved ; whilst the 

 structure of the cellular shells is remarkably perfect, and even 

 the membrane which separates the prisms is in some instances 

 preserved. I have not found shells from the chalk good subjects 

 for microscopic examination : their texture seems to be per- 

 meated by chalky particles, which give it a peculiar opacity, re- 

 sembling that which is seen, from the same eause (occurring in 

 the natural formation of the shell), in Ostrea, Fusus despectus, 

 and other shells distinguished by their opake-white aspect. 



Crustacea. 



In regard to the microscopic structure of the hard envelopes 

 of Crustacea, I can as yet only speak from examination of the 

 common Crab and Lobster ; but the facts which this examination 

 has disclosed are so curious, as to render it desirable to state 

 them in this communication. 



The envelope of the crab and lobster consists of three layers : 



