Deep-water Limpets and Chitons. 17 
of some of the species of Helicina figured by Troschel is quite 
remarkable. This family contains, so far as known, but one 
genus. 
Genus ADpISONIA*, Dall. 
Shell ovate, subconical, strongly asymmetrical, porcellaneous, 
thin ; with a blunt apex curved backward, downward, and to 
the left, without an epidermis; with an unthickened, simple, 
entire margin; pedal muscular impression horseshoe-shaped, 
interrupted in front. Soft parts: head provided with two 
tentacles, without eyes or eye-tubercles ; muzzle plain, simple ; 
foot thin, orbicular, without lateral or posterior tubercles, pro- 
cesses, or fringes; mantle-edge simple, thickened; gill com- 
posed of leaflets as in Patella, the series starting on the right 
behind the head and continued within the mantle-edge back- 
ward, the body of the animal being asymmetrically placed 
with regard to the aperture of the shell to afford room for the 
enormous series of branchial leaflets; anus opening behind 
and above the head slightly to the right of the median line, 
and indicated by a small papilla. 
Radula: see description of the family. 
Type and only species yet known, 
Addisonia paradoxa, n. sp. 
Shell ovate, thin, whitish ; apex presenting an appearance 
as if an embryonic tip (perhaps spiral) had fallen and been 
replaced by a peculiarly blunt ovate apex, which in the young 
shell is nearly marginal, posterior, and to the left of the middle 
line, but in the adult is considerably within the margin, curved 
downward and backward, and much more asymmetrical ; 
sculpture of faint grooves radiating from the (smooth) apex 
and reticulated by the stronger concentric lines of growth, 
besides which the extremely inflated arch of the back is some- 
what obscurely concentrically waved ; over the sculpture the 
shell has a polished appearance ; margins thin, sharp; interior 
smooth, somewhat polished; the scar of the pedal muscle 
narrow, a considerable distance within the margin, the ante- 
rior ends of the scar enlarged, hooked backward on their inner 
edges; these ends connected by a line broadly arched forward 
and marking the attachment of the mantle to the shell 
over the head. Soft parts whitish, dotted with fine purple 
dots; mantle-edge thickened, smooth ; muscular base of the 
* Jn honour of Prof. Addison E. Verrill, of Yale College and the United- 
States Fish Commission, whose surname has already been applied to more 
than one group of invertebrates. 
Ann. & Mag. N. Hist. Ser. 5. Vol. x. 2 
