354 MOLLUSCA. 



is visible; I can find no other organ of respiration than its thin and 

 vascular skin(l). 



ORDER VI. 

 PECTINIBRANCHIATA(2). 



This order forms beyond all comparison the most numerous 

 division, inasmuch as it comprises the whole of the spiral uni- 

 valves; and several that are simply conical. Their branchise, 

 composed of numerous 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 pulmo- 

 nary 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, Cydostoma and Helicina, do we find, 

 instead of branchicB, a vascular network, covering the ceiling 

 of a cavity, that is otherwise similar ; they are the only ones 

 that respire the natural air ; all the others respire water. 



All the Pectinibranchiata have two tentacula and two eyes, 

 sometimes placed on particular pedicles, and a mouth resem- 

 bling a more or less elongated proboscis ; the sexes are sepa- 

 rated. The penis of the male, attached to the right side of 

 the neck, cannot usually be retracted within the body, but is 

 reflected into the cavity of the branchiae ; it is sometimes very 

 stout, and the Paludina is the only one which can retract it 

 through an orifice perforated in its right tentaculum. The 

 rectum and oviduct of the female also run along the right 



(1) These observations are made from individuals presented to me by M. Quoy. 

 M. de Blainville makes a family of Philliroe, which he names Psiliosoma, and 

 which is the third of his AroaoBiiANCHiATA: the others are Ilyalse, &c. 



(2) M. de Blainville's sub-class Pahacephaiophora Dioica. 



