388 GASTEROPODA. 



views which seem to us a great advance on previous 

 classification — we adopt this very natural section, pre- 

 ferring, however, to regard it and the parallel section 

 OpistlbobrancMata as equivalents of the Gasteropoda Pul- 

 monifera. 



The shells of Gasteropoda do not present those dissimi- 

 larities of structure which render the microscopic study of 

 bivalve shells so interesting of late years. They have been 

 carefully investigated by Mr. Bowerbank,* and by Dr. 

 Carpenter,"|" and had previously been inquired into by 

 Mr. Gray. They appear, for the most part, to be made 

 up of three distinct layers, uniformly constituted, but alter- 

 nating in disposition. Each layer is composed of plates, 

 each of which is made up of parallel prismatic cells. Pa- 

 tella, Chiton, and a few other genera, present slight excep- 

 tions to the general rule. In the excellent memoir by 

 Dr. Carpenter, just referred to, a full account of all that 

 has been done on this subject may be found and studied 

 with advantage. 



* Microscopical Society's Transactions, vol. i. 

 t British Association Reports for 1847. 



