TETHYS. 67 



tenth edition of the System a, 1758, as the starting point for binomial 

 nomenclature, no option is left us but to restore the earliest name, 

 Tethys, to this group, and to reject that term from the nomenclature 

 of nudibranchiata. 



The features most depended on for specific characters are (1) the 

 size and degree of union posteriorly of the swimming lobes ; (2) the 

 nature of the mantle-foramen leading to shell-cavity, which may be 

 a large orifice, a minute puncture, or a little tube ; (3) the degree 

 of development of the free posterior lobes of mantle forming the ex- 

 current siphon ; (4) the nature of the opaline gland, grape-bunch- 

 like or scattered, the former having a single external opening, the 

 latter many ; (5) the consistency and form of shell ; and (6) color- 

 pattern (rather than color), and general proportions of animal. 



The structure of the penis will probably be utilized also, in future. 

 The dentition presents slight differential characters, but too small to 

 be of any practical value in discriminating species, so far as pub- 

 lished figures and my own preparations go. Perhaps a wider range 

 of observations will show greater divergence. 



A large amount of work remains to be done before the internal 

 classification of Tethys can be said to approach the standard of 

 present-day zoology. So many species are still imperfectly known, 

 or described merely from the least characteristic organ the shell 

 that any attempt at a natural arrangement of the species now possi- 

 ble will doubtless be subject to much revision in the future. As a 

 preliminary sketch is offered the following : 



Synopsis of Sub genera and Sections. 

 Subgenus TETHYS Linne. 



Body not prolonged backward in an attenuated tail ; sole of foot 

 wide. 



Section Tethys (restricted). 



Swimming lobes ample and free behind as far as their junction 

 with the foot ; opaline gland of the " grape-bunch " type, opening 

 externally by a single orifice ; mantle having a subcentral minute 

 foramen or a little tube communicating with shell cavity; shell 

 with no accessory plate arising within the upper margin. Type T. 

 leporina Linn. 



