CLASSIFICATION. '253 



sight. Ancient autlaors, like Lister, Mtiller, Chemnitz and 

 Schrb'ter, who have treated upon the terrestrial and fluviatile 

 mollusks specially, have rarel}- mistaken these shells ; and a 

 mistake of this nature is ver}' rare among modern authors, 

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

 the shells of terrestrial pulmonates are inoperculate, with entire 

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

 but one cannot give a single character expressible by words, 

 notwithstanding that all who have seen a certain number of 

 species can distinguish them with facilit}^- The fluviatile 

 mollusks, nearly always unicolored, although the}^ ma}^ resemble 

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

 glance. 



Ferussac and several modern authors have thought that all 

 mollusks inhabiting dry land respire by the aid of a pulmonary 

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

 the Littorinas and many trophical Neritinae, live a long while 

 out of water. The larvre of Auricula swim in the sea, and 

 consequently possess a branchial respiration during this period 

 of their life. According to Semper, Ampullaria has an accessory 

 pulmonary sack. If the inoperculate pulmonates are considered, 

 with so much reason, as an incontestable group, it is because, 

 apart from their pulmonary sack, they possess other collateral 

 characters of equal importance, as for example, the position of 

 the e^^es, the organization of the mouth and of the sexual organs. 



The rest of the gastropods, after the exclusion of the pulmo- 

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

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

 heteropods, tectibranchs, scutibranchs, cyclobranchs \ The 

 magniticent work of Quoy and Gaimard is full of precious material 

 for the amelioration of the great group of pectinibranchiates. 

 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 

 animal similar to the Trochidaj, has a nacreous interior, whilst 

 Sigaretus, with a non-nacreous shell, has an animal like Natica. 

 The relations between the enamel of the shell of C^qDrtea, Oliva 

 and Natica, and the structure of the animal were shown for the 

 first time by the same work. As it has become evident to me 

 that the presence of an operculum and the height of the spire, 

 considered heretofore as prime characters, have, in reality, little 

 value in distinguishing the families, I have sought to divine the 

 natural affinities of mollusks according to the sculpture and 

 structure of their shells. 



In 184^, Loven published four plates of lingual dentition, repre- 



