Family APLYSIIDAE. 



Animal lengthened, not protected by a shell, the neck and 

 head narrower than the body ; mouth a vertical fissure ; anterior 

 angles of the head produced into two tentacular lobes folded 

 above; behind them the cylindrical or conical rhinophores, slit 

 above, in front of which are the minute eyes. Parapodia re- 

 curved over the back, forming two lateral or dorsal lobes en- 

 closing the mantle and ctenidium. Genital orifice between the 

 dorsal lobes, communicating by a long furrow with the evertible 

 penis which is near the anterior right tentacle. Shell nearly or 

 entirely covered by the mantle, uncoiled, in the form of a con- 

 cave plate, sometimes absent. Mouth with corneous jaws and a 

 large, multiserial radula composed of similar teeth ; stomach 

 armed with horny nodules ; anus behind the branchial plume. 



Subfamily APLYSIINAE. 



Parapodial lobes well-developed, their anterior ends sep- 

 arated ; genital orifice in front of the branchia ; radula with wide, 

 denticulate, rhachidian teeth, and narrower, serrate and denticu- 

 late laterals. Shell flexible. 



Genus TETHYS Linne, 1758. 



Tethys, Linne, Systema Naturae, loth ed. 1758, p. 653. 

 Laplysia, Linne, Systema Naturae, 12th ed. 1767, p. 1089. 

 Aplysia, Gmelin, Systema Naturae, 13th ed. 1788, I, VI, p. 3103. 

 Tethys, Pilsbry, Proc. Acad. Nat. Sci., Philadelphia, 1895, p. 347. 

 Tethys, Pilsbry, Manual of Conchology, Tryon, XVI, 1896, p. 65. 



Animal swollen behind, narrower in front, with rather long 

 neck and head, bearing folded tentacles and slit rhinophores as 

 usual in the family, the latter about midway between tentacles 

 and dorsal slit. Parapodia arising in front of the middle of the 

 animal's length, ample, freely mobile, free throughout their length, 

 or united for a distance behind, functional as swimming lobes; 

 anterior ends separated. Mantle nearly covering the ctenidium, 

 having a median tube, foramen, or orifice communicating with 

 shell cavity, and produced behind in a more or less developed 



