CLASSIFICATION. 59 



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

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

 The relations between the enamel of the shells of Cypraea, 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 1847 Loven published four plates of lingual dentition, re- 

 presenting 94 species of cephalous mollusks. The first glance 

 at these plates suffices to show clearly that the lingual armature 

 confirms the most of the ancient divisions. Thus, the Cephalo- 

 pods, Pteropods, Heteropods, Scutibranchs (in the sense of Quoy 

 and Gaimard. including the Trochidse), are also distinguished by 

 the teeth. The conchological analogy between Pleurotoma and 

 Conus had already been shown by Sowerby. There existed, 

 nevertheless, certain anomalies until then inexplicable, on ac- 

 count of the small number of observations made, as for example, 

 the analogy between Philine and Scaphander and the Gymno- 

 branchs. The want of teeth must not be considered as fatal to 

 the systematic value of characters found in these organs. The 

 teeth accepted as an exclusive character have, doubtless, incon- 

 veniences, as in the whole animal kingdom, but it cannot be 

 denied that all other organs taken as exclusive characters offer 

 still greater inconveniences. Thus the shell may be wanting in 

 very similar animals (for example, Notarchus, Aplysia ; Ptero- 

 trachea, Cardiopoda ; Limax, Tebennophorus). The operculum 

 is often wanting in the adult, although the young may have it. 

 There are even operculated and inoperculated species in the 

 same genus, as understood by many modern authors (for exam- 

 ple, Pleurotoma and Bela, Oliva and Olivella, Yetus, Yoluta 

 and Lyria, Spirialis and Limacina, Proserpina and Helicina). 

 The organs of respiration and locomotion may be entirely want- 

 ing in closely related species (Firoloidea and Phyllirhoe). 



Loven has characterized the families according to the teeth ? 

 and has given Latin diagnoses. In 1848 Troschel* mentions the 



* Handbook of Zoology, 



