183 



MISCELLANEOUS. 



On the Classification of the Gasteropodous Mollusca. 

 By M. GouRiET. 

 A DIVISION of the Gasteropoda founded on the generative organs 

 presents this radical defect, that many species reputed to be herma- 

 phrodite are constantly being found to be unisexual, and further that 

 Mollusca evidently nearly allied, such as the Helices and Cyclo- 

 stomata, are necessarily separated on the consideration of their sexual 

 organs. 



Therefore most authors have justly selected the respiratory ap- 

 paratus as the basis of classification, since the position of this 

 organ determines the position of the heart and generally that 

 of the anus. In the classifications generally followed, such as 

 that of Cuvier, however, orders are found to be established upon 

 various characters of unequal importance although generally derived 

 from the respiratory apparatus. Thus the Nudibranchs are gene- 

 rally characterized by their uncovered branchiae, although with a 

 restriction for the separation of the Inferobranchs, which are really 

 also Nudibranchs. Elsewhere only the pectinated form of the organs is 

 considered, as in the Pectinibranchs, although in other divisions, such 

 as the Tectibranchs, this form of the branchiae sometimes occurs. 

 The term Tectibranch also conveys to the mind the same sense as 

 Scutibranch, The expression Tubulibranch would seem to indicate 

 a tubular form of the branchiae, when it only refers to the tubular 

 form of the animal. In the case of the Heteropoda the branchiae 

 are set aside, and a character of subordinate value, the form of the 

 foot, is set on the same level as those of the preceding divisions. 

 The word Cyclobranch would perhaps be the most suitable, but for 

 the confusion between them and the Inferobranchs, if taken literally. 



In fact, in the establishment of orders, the position and form of 

 the branchiae, the form of the foot, and the general form of the 

 animal have all been placed in the same rank, without assigning to 

 any one of them a marked preeminence over the others. To remedy 

 this defect the author proposes, taking the respiratory apparatus as 

 his basis, to select the most important of its characters, and to esta- 

 blish the primary divisions upon this. He considers the position of 

 the branchiae to furnish the most important character. The bran- 

 chiae can only occupy three positions : they are either 



Completely external ; 



Or completely internal, and then concealed in a cavity which is 

 itself covered by a shell which usually envelopes the animal ; 



Or simply protected by an imperfect test, a condition intermediate 

 between the two preceding. 



Hence, after the separation of the Pulmonata as a distinct sub- 

 class, we get three great divisions, — the Exobranchs, the Stegi- 

 branchs, and the Endobranchs. 



I. The order of Exobranchs may be subdivided, according to 

 the point of the surface upon which the branchiae are inserted, into — 



