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 
Lagoés thalassios. 'The Romans and medizval 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 animal—survives 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 Proce. Acad. Nat. Sei. Phila., 1895, pp. 847-350 ; but a brief re- 
statement of the facts there brought forward may be useful in this 
place. 
The genus Tethys was founded by Linné in the tenth edition of 
the Systema Nature, p. 653 (1858), for two species, both of which 
are unquestionably sea-hares. 
In the twelfth edition (1767) of the Systema, p. 1089, Linné 
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, pl. 90, f. 15; Fischer, Manuel, p. 
538, pl. 18, £9; Woodward, Manual, pl.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 
