BIVALVIA. 53 
1. Avicuna arcuata, J. Sowerby. Tab. XI, fig. 3. 
Avicuna arcuata. J. Sowerby. Geol. Trans., 2d ser., vol. v, t. 8, fig. 15, 1834. 
_: — Morris. Cat. Brit. Foss., p. 163, 1854. 
Spec. Char. Av. testd elongatd, obliqud, arcuatd, compressa, levigatd, tenui ; valde 
inequilaterali. 
Shell elongate, oblique, and curved, compressed, smooth, thin; very inequilateral. 
Longest diameter, *ths of an inch. 
Locality. Wampstead (Wetherell). 
All that I have seen is the unique specimen in Mr. Wetherell’s cabinet. It. appears 
to be distinguished by its curved form. 4. microptera, Desh. (p. 290, pl. 43, figs. 1S— 
20), resembles itin some characters, but is not so much curved. 
2. Avicuna mEpIA, J. Sowerby. Tab. XI, fig. 1, a—d. 
AVICULA MEDIA. J. Sow. Min. Conch., t. 2. 
— — Prestwich. Geol. Journ., 1847, p. 401. 
— — Morris. Cat. Brit. Foss., p. 163, 1854. 
—  —  Wetherell. Phil. Mag., p. 464, 1836. 
— — D Orb. Prod. de Paléont., t. il, p. 391, 1850. 
Spec. Char. Av. testd inequivalvi, ovato-trigond, valdé obliquda, levigata, fragili ; 
valvd sinistrd converd, tumidd; valvd dextrd depressiore; umbonibus prominentibus, 
remotis ; area cardinalt magno. 
Shell inequivalve, ovately trigonal, very oblique, smooth, and fragile, nacreous ; left 
valve convex and tumid; right valve more depressed; umbo prominent, with a large and 
broad cardinal area. 
Diameter, \ength 2 inches, height 1} inch. 
Localities. a, 6, d, Brockenhurst. c¢, Barton, Bracklesham, Hampstead (Zdwards). 
Highgate, Chalk Farm, Potter’s Bar (Wetherell). Sheppey, Basingstoke, Newnham 
(Prestwich). 
This is by no means rare, and the specimens present a large amount of variation. 
In some individuals, the length of the hinge-line extends so far as to be perpendicular to 
the basal margin on the siphonal side, while im others this sub-auricle, if it may be so 
called, is considerably shorter. The Barton specimens are generally the more oblique, 
though not always so; the specimen figured from Brockenhurst is somewhat quadrate, 
and corresponds in that character with the specimens figured in ‘Min. Conch.,’ from 
Highgate. It is seldom that the outer surface is preserved, but in one specimen 
