528 PEOF. C. H. o'dONOGHUE ; REPORT ON 
Genus Tethts Linn. 1758. 
Type by designation, T. leporina Linn. ; T. Umacina Linn, 
being unidentifiable. 
Synonymy : Aplysia Linn., 1767, and subsequent authors. 
Type. Laplysia depilans Linn. 
The animal is swollen beliind, narrower in front, with rather a long neck 
and head, bearing folded tentacles and slit rhinophores as is usual in the 
family ; the latter lie about midway between tentacles and dorsal slit. The 
pleuropodia arise in front of the middle of the animal's length, are ample, 
freely mobile, free throughout their length or united for a distance behind. 
They are functional as swimming-lobes ; their anterior ends are separated. 
The mantle nearly covers the gill, having a median tube, foramen, or orifice 
communicating with shell-cavity, and produced behind in a more or less 
developed lobe or lobes, folded to form an excurrent siphon. The genital 
orifice lies under the front edge of the mantle, in front of the gill ; an 
opaline gland is present a short distance behind the genital opening. The 
foot is well developed. The shell is very thin and membranous with a thin 
calcareous inner layer; it is nearly as large as the mantle, concave, with 
pointed small apex, bears a recurved lamina, and has a concave posterior sinus. 
I am aware that it is an extremely difficult matter in certain cases to 
identify members of the genus Tetliys by their shells, which are on the whole 
very similar, but the two following species have been placed under the same 
names as j)reviously recoi'ded, but poorly described, forms. This has been 
done because I have had the opportunity to examine the type-material and in 
neither case have I been able to detect significant differences between the 
present and the previously described species. Further, the localities are in 
each instance sufficiently near to make it probable that they fall within the 
limits of the distribution of the respective species. Literature, moreover, is 
strewn with nomina nuda due to the multiplication of names founded upon 
unsatisfactory descriptions or without sufficient notice of previous work. 
Species Tethys gigantea (Sowerby), 1869, Conch. Icon. vol. xvii., Aplysia, 
pi. 1, sp. 1, fig. la&b. (PI. 27. fig. 1. PI. 29. figs. 20-22.) 
Body. The animal is vei-y large, quite justifying its name of gigantea, and 
its body is plump, high, and somewhat egg-shaped. The visceral hump is 
placed relatively far back in the largest part of the body, which narrows and 
gets lower as it passes forwards to the cephalic region. The very well- 
developed pleuropodial lobes arise a short distance behind the rhinophores, 
are freely and widely separable, and unite posteriori}' to form a transverse 
fold across the hinder end of the body above the short but well-marked 
caudal prolongation of the foot. The moderately thick mantle is perforated 
by a small aperture over the centre of the shell. It covers the ctenidium, 
save for a narrow strip postero-laterally, but more completely than in the 
