94 ONCIDIID.E. 



quadrangular, scarcely covering the head and foot, set with 

 small conical papillae : do?sal tentacles subclavate, stout, with 

 smooth, slightly elevated sheaths : head-veil broad, undulating : 

 gills 7, large, spreading transversely : foot large and thick. 

 L. 1. 



Habitat : A single specimen was dredged near Berry Head 

 in 1845 (Alder) ; and another has since been got at Hilbro 

 Island, mouth of the Dee, Cheshire, by Mr. Byerley. 



Order VI. PULMONOBEANCHIATA, Sowerby. 



(See Vol. I. p. 76, and Vol. III. p. 200.) 



Bibliopolical reasons caused an interruption of the 

 natural classification or arrangement of the British 

 Mollusca in the present work. The students of land 

 and freshwater shells are so numerous compared with 

 those who collect marine shells, owing chiefly to the 

 want of opportunities, that the publisher considered 

 it expedient to divide the subject and have separate 

 volumes for each branch. But it is evident that this is 

 an artificial barrier, and that the Mollusca ought to be 

 studied without regard to the nature of their habitat. 



The air-breathing Mollusca which inhabit the sea- 

 shore and tidal estuaries are few in number. They may 

 be called amphibious, inasmuch as they live partly in 

 water and partly on land. None have been discovered 

 in latitudes more northern than our own. 



Family I. ONCIDHD^, (Onchidiidce) 

 H. & A. Adams. 



Body slug-like, coriaceous : tentacles 2, conical or cylindri- 

 cal : eyes placed near the extremities or tips of the tentacles : 



