1898.] NATURAL SCIENCES OF PHILADELPHIA. 193 



DESCRIPTION OF A NEW TETHYS (APLYSIA). 

 BY E. J. LETSON. 

 Tethys Pilsbryi n. sp. Plate VIII. 



Length 11 £ cm., body elongated, flabby, plump, enlarged behind. 

 Mouth encircled by wide lips and large buccal appendages. Rhino- 

 phores (posterior tentacles,) stout, short conic, slit at the extremities ; 

 eyes small, black, placed before the rhinophores. Anterior ends of 

 pleuropodial lobes well separated, posterior ends joined behind, 

 mantle large, median perforation very small, surrounded by radial 

 striae (visible only under a lens). Right margin of mantle, folded 

 over about half its width, and largely adnate ; posteriorly not 

 forming an excurrent siphon ; gill considerably exposed. Genital 

 opening under forward right border of mantle, surmounted by a 

 fleshy prominence. Opening of opaline gland large, single, about 

 13 mill, back of the genital pore, and well under the gill. Gills 

 foliated in regular branches. External integument smooth; with a 

 few inconspicuous scattered warts ; olive colored, with some cloud- 

 ing of black on the reflexed mantle and sometimes also near the 

 tail ; shell normal. 



Silam, North Coast of Yucatan, (Heilprin). 



This differs from all other known species in having the mantle 

 folded back upon itself and adnate except near the edge. 



