588 HISTORY OF SYSTEMS. 



The families are 1. Naticidse ; 2. Littorinidae ; 3. Trun- 

 catellidae ; 4. Paludinidae ; 5. Velutinidae ; 6. Pyra- 

 midellidae ; 7. Tornatellidae. 



** Branchiae formed of long filaments, and often pro- 

 truded when the animal is expanded. Shell various 

 and anomalous in form, with often a very large aper- 

 ture. 



The families are 1. Valvatidae ; 2. Vermetidae ; 3. Vani- 

 coroidae ; 4. Capulidae ; 5. Crepidulidae ; 6. Phoridae. 

 II. HETEROBRANCHIATA. Branchiae variously form- 

 ed, or respiring by means of lungs. 



Order III. PLEUROBRANCHIATA. Branchiae lamellar 

 placed on the right side of the back, and covered with 

 a thin mantle, which is sometimes protected by a 

 small shell, more or less sunk within its substance. 



* Branchiae on the side of the back and covered by the 

 mantle. 



The families are 1. Bullidae ; 2. Aplysiadae ; and per- 

 haps, 3. Pterotracheidae ; 4. Argonauta, and 5. Belle- 

 rophon. 



** Branchiae on the right side of the body, in the groove 

 between the edge of the mantle and the foot. 



The families are 1. Pleurobranchidae ; 2. Umbrellidae. 



Order IV. GYMNOBRANCHIATA. Branchiae naked, vari- 

 ous, placed on different parts of the back, or a series of 

 plates placed round the edge of the mantle. 



* Branchiae placed on the back and exposed. 



The families are 1. Doridee ; 2. Tritoniadae ; 3. Placo- 



branchidae. 

 ** Branchiae lamellar, on the edge of the under side of 



the mantle. 

 The families are 1 . Phyllidiadae ; 2. Patellidae ; 3. Chi- 



tonidae. 

 Order V. PNEUMOBRANCHIATA. Respiring free air. 



* The respiratory cavity closed by the edge of the mantle 



being attached to the back of the neck, leaving only a 

 small hole, covered with a fleshy valve, for the entrance 

 and escape of air. No operculum. 



A. Eyes on the top of the long cylindrical tentacula. 

 The families are 1. Arionidae ; 2. Helicidae ; 3. Veroni- 



cellidae ; 4. Onchidiadae. 



B. Eyes placed at the base of the tentacula. 

 The families are 1. Auriculidae ; 2. Limnaeadse. 



c. No distinct tentacula. 



The families are 1. Amphibolidae ; 2. Siphonariadae ; 3. 

 Gadiniadae. 



