174 Binney on the Naked Air-Breathing Mollusca. 



genera included in the same division, were supposed to pos- 

 sess only contractile tentacles, by a convenient method of 

 generalization he inferred that in the present species also the 

 tentacles were destitute of the power of retraction. The 

 editors of the new edition of Lamarck have again more re- 

 cently described it as destitute of a mantle, but in truth it 

 possesses a well characterized mantle, unattached to the body 

 at its anterior part, and around its whole margin, and as before 

 remarked, the tentacles are retractile. M. Ferussac also spec- 

 ulated upon the uses of this peculiar organization, which he 

 supposed might enable the animal to resist the heats of warm 

 climates ; it is, however, probably intended for some other 

 purpose, for it is found to inhabit the most northern portions of 

 the United States, where the winters are long and severe, and 

 by its habits it is in a great degree removed from the influ- 

 ence of heat. 



It may be noticed that the description of Dr. Gould differs 

 essentially from mine ; this 2irises from his description having 

 been drawn up from specimens preserved in alcohol, which 

 had contracted them and entirely changed their aspect. 



Genus Philomycus, Rafinesque. 



PHILOMYCUS DORSALIS. 



P, corpore cylindraceo, postice attenuato ; dorso linea longitudinali nigres- 

 cente interrupta et glandulis minutis longulis instructo ; clypeo nullo; apertura 

 lateral! parvo antica. 



SYNONYMES AND REFERENCES. 



Philomycus dorsalis, nobis. 



DESCRIPTION. 



Color of upper surface ashy, with a shade of blue, an inter- 

 rupted black line extending down the centre of the back ; 

 superior tentacles black, about one eighth of the length of 

 the body : lower tentacles blackish, very short. Body cylin- 

 drical and narrow, terminating posteriorly in an acute point ; 

 base of foot white, very narrow, its separation from the body 

 not well defined. Upper surface covered with elongated and 



