66 TETHYS. 



Shell very thin, membranous with a thin calcareous inner layer, 

 nearly as large as mantle, concave, with pointed, small apex, bearing 

 a recurved lamina, and having a concave posterior sinus. 



Distribution : all tropical and warm temperate seas. 



A reference to the table of genera on p. 64, will show the general 

 relations this genus bears toward other genera of the family. 



Species of Tethys have been known and noticed in the literature 

 of the precocious Mediterranean peoples from very early times. The 

 resemblance to a land mammal commemorated in the English com- 

 mon name, Sea-Hare, was first noticed by the Greeks, who called it 

 Lagoos thalassios. The Romans and mediaeval writers paraphrased 

 this in Lepus marinus ; and the French vernacular Lievre de mer, 

 the Italian Lepre marina, etc., retain the same idea. Some other 

 French names for the slabby beast, more appropriate than polite, 

 are given by Rang. The natural history compilers of the Roman 

 and Middle Age periods, collected all sorts of absurd popular stories 

 about the dangerous and deadly qualities of Aplysia; for the water- 

 side folk the world over usually consider any uneatable animal as 

 dangerous or poisonous. The memory of one of these tales that 

 baldness resulted from handling the animalsurvives in the name 

 of one of the species, depilans. The nauseous odor of the living 

 animal may have something to do with its ill repute. 



Aplysias not only crawl with facility, but the typical species swim 

 freely and rapidly by means of a wing-like motion of the pleuro- 

 podia or " swimming lobes." 



The generic name of the genus has been discussed by the writer 

 in Proc. Acad. Nat. Sci. Phila., 1895, pp. 347-350 ; but a brief re- 

 statement of the facts there brought forward may be useful in this 

 place. 



The genus Tethys was founded by Linne in the tenth edition of 

 the Systema Naturae, p. 653 (1858), for two species, both of which 

 are unquestionably sea-hares. 



In the twelfth edition (1767) of the Systema, p. 1089, Linne* 

 wholly alters the diagnosis of Tethys, applying that generic name to 

 the Nudibranch still known as Tethys (see Tryon, Structural and 

 Systematic Conchology, ii, p. 381, pi. 90, f. 15 ; Fischer, Manuel, p. 

 533, pi. 13, f. 9 ; Woodward, Manual, pi. 13, f. 9). In this edition 

 of the Systema, a new name, Aplysia or Laplysia, is proposed for the 

 sea-hares. It would seem, therefore, that if we are to adopt the 



