OPISTHOBRANCHIATA FROM THE ABEOLHOX ISLANDS. 527 
discarded. The asymmetrical position of the ctenidium involves also a 
certain asjMnmetry of the viscera that is not characteristic of the Nudi- 
branchiata. 
The main sub-divisions employed here are those suggested by Fischer in 
1884 (41) and adopted by Vayssiere (74) and Pilsbry (67) among others, and 
the sub-order is split into three divisions : (1) Cephalaspidea, (2) Anaspidea, and 
(3) Notaspidea. Three of the species in the collection belong to the second 
group (the Anaspidea) and fall within the one family (the Tethyidpe). The 
other belongs to the last group, the Notaspidea. 
(2) TECTIBEANCHIATA ANASPIDEA Fischer, 1884. 
Tectibranchs that lack a fleshy head-shield ; the head possesses two or 
four folded or split tentacles ; the shell is spiral or plate-like, usually hidden 
by mantle, and with a posterior terminal nucleus, rarely absent ; the penis 
is near right anterior tentacle, the female aperture and vas deferens open 
near gill. 
Family TETHYIDiE (Aplxsiid^,)- 
The animal is elongate ; the shell is in the form of an uncoiled concave 
plate, almost or entirely hidden, or absent; the neck and head are narrower 
than the body, the mouth is a vertical fissure ; the epipodia or pleuropodia 
are recurved over back, forming two lateral or dorsal lobes enclosing mantle 
and gill. The genital orifice lies within dorsal slit, communicating by a long- 
furrow with the invertible penis, which lies near the anterior right tentacle ; 
the mouth is armed with horny jaws ; the multiserial radula possesses 
similar teeth ; the stomach is armed with cartilaginous nodules ; the anus is 
situated behind the gill. 
The name generally used for the family is Aplysiidae from the generic 
name A-plysia, but Linna3us in his tenth edition, 1758, uses the term Tetliys 
for an animal which he briefly defines and refers to a figure in Rondelet, and 
this is bej'-ond doubt the ordinary sea-hare usually referred to worn 2iS Aplysia. 
Later, in the twelfth edition, 1767, he applies the term Aplysia or Laplysia 
to the very same creature, and uses the name Tethys for the Nudil)ranch 
commonly known as Tethys fimhria. Pilsbry has dealt fully with this 
matter (66), and points out, quite correctly, that the term Tethys is the 
proper one for this genus and not applicable to Nudibranchs. He adopts 
this nomenclature in his monograph (67, p. 59), but strangelj^ enough retains 
the old family-name Aplysiidse, whereas the family should take its name 
from the oldest or typical genus of the family The correct family-name, 
however, is used by Hedley (50, p. 107), 1918. There is no doubt that Pilsbry 
was right in making this change, and the genus is Tetliys with AjAysia as a 
synonjan. It is to be regretted that earlier writers did not go back to the 
original literature, but continued to use the word Aplysia and misapply 
the term Tethys. 
