52 



We are not so certain that we should place there the 



Phylliroe, Perov., 



An du Mas., XV, pi. ii, f. 1, where the transparent and strongly com- 

 pressed body has a snout before, surmounted by two long tentacula 

 without eyes, a truncated tail behind, and which allows the heart, 

 nervous system, genital organs of botli sexes to be seen through the 

 integuments. The genital orifices and that of the anus are on the 

 right side, and sometimes a tolerably long penis is visible ; I can find 

 no other organ of respiration than its thin and vascular skin*. 



ORDER VI. 

 PECTINIBRANCHIATAf. 



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 branchiae, composed of nu- 

 merous lamelleg 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 a shell, and which has a large opening between the 

 edge of the mantle and the body. 



In two genera only, Ci/clostoma and Helicina, do we find, instead 

 of branchise, a vascular netAVork, covering the ceiling of a cavity, in 

 other respects very similar ; they are the only ones that respire the 

 natural air ; all the others respire water. 



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

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

 or less elongated proboscis; the sexes are separated. The penis of 

 the male, attached to the right side of the neck, cannot usually be re- 

 tracted within the body, but is reflected into the cavity of the branchiae; 

 it is sometimes very stout, and the Paludina is the only one Avhich 

 can retract it through an orifice perforated in its right tcntaculum. 

 The rectum and oviduct of the female also creep along the right side 

 of the cavity, between them and the branchiae is a peculiar organ 

 composed of cells, from which exudes an extremely viscid fluid ; 

 tliis forms a common envelope which contains the ova, and which is 



* These observations are made from individuals presented to me ly M. Quoy. 

 M. de Blainville makes a family of Philliroe, which he names PsiUosoma, and v.hlch 

 is the third of his Aporubrunchiata : the others are Hyala?, &c. 



-f- M. de Blaiuville's sub-class Paracephalophoru Diuica. 



