58 CLASSIFICATION. 



cult to indicate by a description the difference which exists 

 between the shell of a pulmonate and that of a branchiate mol- 

 lusk, there are, nevertheless, few collectors who will not recog- 

 nize it at sight. Ancient authors, like Lister, Miiller, Chemnitz 

 and Schroter, who have treated upon the terrestrial and fluviatile 

 mollusks specially, have rarely mistaken these shells ; and a 

 mistake of this nature is very rare among modern authors, 

 although a few instances might be cited. One can say only that 

 the shells of terrestrial pulrnonates are inoperculate, with entire 

 apertures (holostomate), never nacreous, rarely spirally striated; 

 but one cannot give a single character expressible by words, not- 

 withstanding that all who have seen a certain number of species 

 can distinguish them with facility. The fluviatile mollusks, 

 nearly always unicolored, although they may resemble marine 

 groups as to form, can also be readily separated at a glance. 



Ferrussac and several modern authors have thought that all 

 mollusks inhabiting dry land respire by the aid of a pulmonar}- 

 sack, but nothing is less certain. Among branchiferous genera, 

 the Littorinas and many trophical Neritina*, live a long while 

 out of water. Tbe larvae of Auricula swim in the sea, and con- 

 sequently possess a branchial respiration during this period of 

 their life. According to Semper, Ainpullaria has an accessory 

 pulmonary sack. If the inoperculate pulmonates are consid- 

 ered, with so much reason, as an incontestable group, it is be- 

 cause, apart from their pulmonary sack, they possess other 

 collateral characters of equal importance, as for example, the 

 position of the eyes, the organization of the month and of the 

 sexual organs. 



The rest of the gasteropods. after the exclusion of the pulmo- 

 nates, were divided by Cuvier into several very natural groups, 

 according to the form and position of the branchire (nudibranchs, 

 heteropods, tectibranchs, scutibranchs, cyclobranchs). The 

 magnificent work of Quoy and Gaimard is full of precious ma- 

 terial for the amelioration of the great group of pectinibranchi- 

 ates. The Trochidse are here shown to be inseparable by their 

 characters, as much external as internal, from Haliotis and other 

 scutibranchs, notwithstanding the presence of an operculum and 

 an elevated spire in the former. At the same time the great 

 systematic value of the nacre was proved. Stomatella, with an 



