and Gasteropods, and of course between those Odontophorons 

 Mollusks and the Conchifers. The Pulmonifers of Cuvier by 

 some considered as a class apart and the Solenoconchs by 

 some considered as also entitled to classic rank, by others re- 

 ferred to the Pteropods, and by others still to the Conchifers 

 have also been retained as sub-classes of the Gasteropods. The 

 classification thus accepted is then the same as those already 

 proposed, in 1861, by Prof. Dana 1 in his "Manual of Geology," 

 and, in 1865, by Prof. E. S. Morse in his " Classification of the 

 Mollusca based on the principle of cephalization." So far as 

 the combination of the Pteropods, Heteropods, and typical Gas- 

 teropods into one class, others had also long before indicated the 

 propriety of the innovation. The other groups regarded as of 

 approximately equal value with those, and therefore designated 

 sub-classes, are the Pectinibranchiates and Opisthobranchiates. 



i 



ORDERS. 



Applying to the combinations of the Gasteropods into orders 

 the principle that morphology and not teleology is the guide in 

 natural classification, it becomes necessary to depart from some 

 quite generally accepted schemes, and especially that whereby 

 all the air-breathing rnollusks are combined together in contra- 

 distinction from those respiring by means of branchiae. As was 

 perceived long ago by Cuvier, the inoperculated Pulmonifers 

 (except Proserpinidse) are entirely different from the operculated 

 ones. That great naturalist very justly retained alone in one 

 group the former (the Proserpinidse were unknown to him), and 

 thus constituted a truly natural order, while the operculated ones 

 (Cyclostomse, etc.) were referred to the Pectinibranchiates, and 

 near Littorina, with which the best naturalists still associate them. 

 His ignorance of the structure of the Helicinidee induced him to 

 retain them near the Cyclostomse, but had he been acquainted 

 with tnem, he would doubtless have combined them with his Tro- 

 choidea as they now are. The combination of all the Pulmoni- 

 ferous Gasteropods into one group, as was afterwards done, was 



1 Prof. Dana has only differed in the depreciation of the value of the 

 primary groups, the Mollusca (his ordinary Mollusca) and the Molluscoidea 

 (his Antlwid Mollusca) being considered as classes, and their subdivisions 

 as orders. 



