64 CLASSIFICATION. 



1. Trunk not retractile. . 



2. Trunk retractile onlj^ by the end. 



3. Trunk retractile from the base. 



These differences appear to originate solely in the different 

 length of the trunk. The old divisions Holostomata and Entos- 

 tomata are not very faulty. 



Troschel's first section commences with the Pulmonates, but 

 Morch considers very doubtful their having a true pulmonary 

 sack closed by a contractile opening. Gray calls these respira- 

 tory organs of the Cyelostomae " gills vascular, branched," and 

 '' gills indistinct in the form of series of vessels on the inner sur- 

 face of the mantle.'' 



Sect. 1. Fam. 1. Aciculncen ; 2. Pomatiacea ; 8. fyclotacea ; 

 4. Oyclostomacea. 



Sect. 2. Respiring by branchia^ and lungs ; AmpuUariacea. 



Sect. S. Branchiferous Holostomata. Fam. 1. Valvatse ; 2. 

 Hydrobise (Lithoglyphus' ; 3. Littorinida? ; 4. Rissoidae ; 5. Pa- 

 ludinidae ; (3. Melaniidae ; 7. Potamidai ; 8. Cerithiida? (Planaxis) 



The Aporrhaidjie form a passage between the Cerithiidae and 

 Strombidie. Crepidula and Capulus belong with Hipi)onyx in a 

 group, notwithstanding differences in the labial palpi. Onus- 

 tidiB connects the Crepidulida^ with the Heteropods. 



Ovulidse (including Pedicularia), generally placed close to Cy- 

 pi'sea, is strongly distinguished from the latter by its non-retrac- 

 tile proboscis. Notwithstanding this character, Miirch considers 

 it intermediate between Cassis and Cyprjea. 



The 3d section of Troschel (trunk retractile from the base) 

 contains the genera which Mfirch united in 1852 in the family 

 Tritonidae, namely : Cassis, Dolium, Pyrula, Triton, Ranella. 



Onchidiopsis, Velutina, Marsenia, Tylodina, form, probably, a 

 very natural division, notwithstanding the want of the two lateral 

 teeth in Mai'senia. The family Naticidae stands next in relation- 

 ship. It is in the Tsenioglossata that the greatest incertitude 

 relative to a natural grouping of the families exists ; in the orders 

 which follow, these relationships are more i^ositively defined. 



Order 2. Rhachiglossata, Troschel. Never more than 

 three rows of teeth. All mollusks having coriaceous ovisacs, 

 heretofore known, belong to tliis order. 



