BIVALVIA. 291 
1. GuycimerRis anGusTA, Wyst and Westendorp. Tab. XXIX, fig. 2, a—d. 
Giycrmerts aNGusta. Nyst and West. Nouv. Rech. Coq. Foss. d’Anv. (Bull. de l’Acad. 
des Se. de Bruxelles, tom. vi, 1839), p. 4, pl. 1, fig. 1. 
—_ — Nyst. Coq. Foss. de Belg., p. 55, pl. 11, fig. 1, 1844. 
— vacina. §. Wood. Catal. of Crag Shells in Ann. and Mag. Nat. Hist., vol. vi, 
p. 245, 1840. 
—_— — J. Sow. Min. Conch., t. 636, 1844. 
Spec. Char. Testi elongato-oblongd. crassd, rugosd, tortd, valde inequilateral ; pos- 
ticé breviore, truncata, subangulatd ; antice productd, attenuata ; valvis intus incrassatis. 
Shell elongately oblong, thick, rough and twisted, and very inequilateral; pos- 
terior side the shorter, truncated, and somewhat angular; anterior produced, slightly 
pointed; valves thickened within. 
Length, 4 inches. Height, 1} inch. 
Locality. Cor. Crag, Gedgrave, Sudbourn, and Ramsholt. 
Red Crag, Sutton. 
This strong shell is by no means rare in the Coralline Crag, and fragments of it are 
not unfrequently met with in the Red Crag. Some differences exist between this and 
the recent species, Mya siliqua, Chemn., vol. ii, p. 192, t. 198, fig. 1934; the siphonal 
side being shorter in the Crag shell, as well as broader, and the anterior more pointed ; 
there is also a greater twist or contortion in the valves, and the muscular impressions 
are somewhat different; that of the anterior adductor is more elongated, increas- 
ing in breadth towards the anterior, and extending to the verge of the impression by 
the mantle on that side; the posterior adductor is situated further backward than in 
the recent shell, almost touching the extreme edge, below which is the small sinus of 
the mantle-mark. The shell is much twisted, so that the valves, when united, touch 
only at the hinge and basal edge of the anterior margin. There is a large and pro- 
minent fulcrum for the ligament, extending nearly to the posterior extremity of the 
shell; between it and the umbo is a cavity for the cartilage, with a callosity or obscure 
tooth immediately under the beak. 
There is a slight appearance of erosion at the umbones of some of my specimens, 
and the exterior is smooth, with the exception of lines of growth. Traces of irregular 
lines may be sometimes seen both longitudinally and transverse; these were probably 
produced by the rugosities of its thick epidermis. 
In this, contrary to the generality of Bivalves, the siphonal side is much the shorter 
of the two. 
A recent species of this Genus is in the cabinet of my friend J. W. Flower, Esq., 
said to be from Moreton Bay. It approaches rather nearer to our fossil than does the 
Arctic shell, in having a greater twist, but it differs also slightly in form. 
