BIYALYIA. 



1. Avicula arcuata, J. Sowcrbg. Tab. XI, fig. 3. 



Avicula arcuata. J. Sowerly. Gcol. Trans., 2d ser., vol. v, t. 8, fig. 15, 1834. 

 — — Morris. Cat. Brit. Fobs., p. 1C3, 1854. 



Spec. Char. Av. testa clongatd, obliqud, arcuata, compressd, laevigata, tenui; valde 

 inaquilaterali. 



Shell elongate, oblique, and curved, compressed, smooth, thin; very inequilateral. 



Longest diameter, fths of an inch. 



Local itg. Hampstead, ( Wetherett) . 



All that I have seen is the unique specimen in Mr. Wetherell's cabinet. It appears 

 to be distinguished by its curved form. A. microptera, Desh. (p. 290, pi. 43, figs. 18 — 

 20), resembles it in some characters, but is not so much curved. 



2. Avicula media, /. Sowerbg. Tab. XI, fig. 1, a — d. 



Avicula media. J. Sow. Min. Conch., t. 2. 



— — Prestwich. Gcol. Journ., 1S4", p. 401. 

 — Morris. Cat. Brit. Foss., p. 1G3, 1854. 



_ _ Wetherell. Phil. Mag., p. 464, 1836. 



— — VOrb. Prod, de Paleont., t. ii, p. 391, 1850. 



Spec. Char. Av. testa, ina?quivalvi, ovato-trigond, valde obliqud, lavigatd, fragili ; 

 valcd sinistra convexd, tuniidd; valvd dextrd depressiore ; u/nbouibus prominentias, 

 remolis ; area cardinali 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, length 2 inches, height \\ inch. 



Localities, a, b, d, Brockenhurst. c, Barton, Bracklesham, Hampstead {Edwards). 

 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 in 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 



