64 CLASSIFICATION. 



1. Trunk not retractile. 



2. Trunk retractile only 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 Cyclostomse " 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. Aciculacea ; 2. Pomatiacea ; 8. Cyclotacea ; 

 4. Cyclostomacea. 



Sect. 2. Respiring by branchiae and lungs ; Ampullariacea. 



Sect. 3. Branchiferous Holostomata. Fam. 1. Valvatae ; 2. 

 Hydrobiye (Lithoglyphus^ ; 3. Littorinidae ; 4. Rissoidse ; 5. Pa- 

 ludinidae ; 6. Meianiidse ; 7. Potamidae ; 8. Cerithiidae (Planaxis) 



The Aporrhaidse form a passage between the Cerithiidae and 

 Strombidee. Crepidula and Capulus belong with Hipponyx in a 

 group, notwithstanding differences in the labial palpi. Onus- 

 tidse connects the Crepidalidfe with the Heteropods. 



Ovulidae (including Pedicularia). generally placed close to Cy- 

 praea, is strongly distinguished from the latter by its non-retrac- 

 tile proboscis. Notwithstanding this character, Morch considers 

 it intermediate between Cassis and Cyprsea. 



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

 contains the genera which Morch united in 1852 in the family 

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



Onchidiopsis, Yelutina, Marsenia, Tylodina, form, probably, a 

 very natural division, notwithstanding the want of the two lateral 

 teeth in Marsenia. The family Naticidae stands next in relation- 

 ship. It is in the Taenioglossata that the greatest incertitude 

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

 which follow, these relationships are more positively defined. 



Order 2. RHACHIGLOSSATA, Troschel. Never more than 

 three rows of teeth. All mollusks having coriaceous ovisacs, 

 heretofore known, belong to this order. 



