FAMILY ONCHIDIID^E 113 



whole, with additions, is now given so that the data in regard to this 

 species may be obtained in one place. 



The animal lives between tides, where at high water it is covered 

 by the sea, usually on stones or projecting rocks, either where it is cov- 

 ered with Fucus or on the underside of stones which thus form a 

 shelter. It seems to be gregarious in its habits, as many as fifty speci- 

 mens having been taken from a single crevice in shaly rock. When in 

 motion it moves quite rapidly for so small an animal, with two short 

 stout tentacles tipped by keen black eyes protruding beyond the front 

 edge of the mantle. The upper surface is dark slate color, with spots 

 or streaks of light gray or whitish. It appears smooth, but as if hav- 

 ing small round tubercles beneath the smooth skin, which when the 

 animal is contracted in alcohol are much more conspicuous than in life. 

 Around the edge of the mantle is a single row of larger and more 

 prominent tubercles corresponding to an equal number of mucous glands. 

 These, projecting, give the margin a serrate or fringed appearance. 

 The animal, when in motion, is about twelve millimeters long, four 

 and one half wide, and three millimeters high, oblong oval in form, a 

 little wider behind than in front. When at rest in a contracted state 

 it is nearly circular in form, a little longer than wide, the center of 

 the dorsum elevated in a bluntly pointed manner, giving the creature 

 much the aspect of a young Acmcea. The lower surface of the body 

 is of a greenish white, and, when the animal is moving, the foot 

 seems to undergo rapid undulation. The muzzle exhibits anterior 

 ovate extensions separated by a sulcus in the median line, as in other 

 species of the genus. 



Neither Onchidella borealis nor O. celtica possesses the singular 

 dorsal eyes characteristic of many tropical species. 



O. borealis differs from O. carpenteri Binney, and all the other 

 species of the family now known (except O. celticum) , in possessing 

 a thin delicate smooth jaw, the presence of which has been demon- 

 strated by both Binney and Semper. According to Joyeaux Laffluie 

 O. celticum also possesses a jaw, though the surface of the dorsum is, 

 if the figure given by Forbes and Hanley be accurate, much more 

 prominently tuberculous than in O. borealis. O. carpenteri Bin- 

 ney, a small species reported by Binney from California and Puget 

 Sound, is according to that author agnathous, and therefore belongs to 

 the typical section of the genus. 



The dentition of O. borealis has been worked out by Binney and 

 confirmed by Semper. The radula is long and wide, the teeth arranged 

 strongly en chevron, with a formula of ty % ty. The rhachidian 



