50 INTRODUCTION. 



have patelliform, open, and scarcely spiral shells ; the Trache- 

 lipoda with the foot distinct and attached to the neck of the 

 animal, have spiral, non-symmetrical shells. The Cephalo- 

 poda, with arms covered by suckers surrounding the head of 

 the animal, have generally symmetrical convolute shells. The 

 Cephalopoda are divided into C. polythalamia, which have 

 the internal cavity divided into chambers by septa, as in the 

 Nautilus ; and the C. Monothalamia, which are not so divided, 

 as the Argonauta. The order Heteropoda contains the genus 

 Carinaria alone. 



Order Pteropoda. 



This order, containing hyaline, symmetrical, non-spiral shells, 

 as above described, is not divided into families, but contains 

 the following genera, Hyalsea, Cleodora, Limacina, Cymbulia ; 

 the first of which, although composed of a single piece, resem- 

 bles a bivalve so nearly, that Linnaeus actually placed it in his 

 genus Anomia. 



Order Gasteropoda. 



With the exception of the genus Bulla and Vitrina, the last 

 of which forms a passage into the next order, the shells con- 

 tained in this order are patelliform, open, and scarcely spiral. 

 They are divided into the following families : — 



1. Phyllidiana (plates, fig. 227 to 231), containing the 



genera Chiton, Chitonellus, and Patella, the two former 

 of which present the only exception to the statement 

 above made, that all the shells of Mollusca were uni- 

 valve. 



2. Semiphy llidiana (plates, fig. 232 and 233). Of the two 



genera contained in this family, Pleurobranchus is broad, 

 thin, and slightly spiral at the apex, and Umbrella is 

 flat, circular, with a central apex. 



