GASTEROPODA HETEROPODA. 259 



faculty of distending their body by filling it with water, in a way 

 not well understood. Forskahl comprised them all in his genus 



Pterotrachea, 



Which is now divided into Carinaria, Atlanta, Flrola, Timorienna, Mono- 

 phora, PhylUroe. The Argonauta vitrea of authors must be the shell of a 

 Carinaria, but the animal is not yet known. 



ORDER VI. 

 PECTINIBRANCHIATA. 



This order forms, beyond all comparison, the most numerous divi- 

 sion, inasmuch as it comprises the whole of the spiral univalves, and 

 several that are simply conical. Their branchm composed of nu- 

 merous lamellae or strips laid parallel with each other, like the teeth 

 of a comb, are attached on one, two, or three lines, according to 

 the genus, to the ceiling of the pulmonary cavity, which occupies 

 the last whorl of the shell, and which has a large opening between 

 the edge of the mantle and the body. 



In two genera only, Cyclostoma and Helicina, do we find, instead 

 of branchiae, a vascular net-work, covering the ceiling of a cavity, 

 that is otherwise similar; they are the only ones that respire the na- 

 tural air; all the others respire water. 



All the Pectinibranchiata have two tentacula and two eyes, some- 

 times placed on particular pedicles, and a mouth resembling a more 

 or less elongated proboscis. 



Their tongue is armed with little hooks, and by slow and repeated 

 rubbing acts upon the hardest bodies. 



The greatest difference in these animals consists in the presence 

 or absence of the little canal formed by a prolongation of the edge 

 of the pulmonary cavity of the left side, and which passes through a 

 similar canal or emargination in the shell, to enable the animal to 

 breathe without leaving its shelter. There is also this distinction 

 between the genera — some of them have no operculum; the species 

 differ from each other by the filaments, fringes, and other ornaments 

 of the head, foot, or mantle. 



These Mollusca are arranged in several families according to the 



