BIVALVIA. . 81 
Spec. Char. A. testd variabili, plerumqué oblongd, angustatd, subtetragond, sub- 
obliqua, valdé inequilaterali, striata; umbonibus distantibus, recurvis ; siphoni-regione 
productd, biangulata ; striis radiantibus, numerosis, squamoso-granulatis. 
Shell variable, for the most part elongately oblong ; somewhat oblique, very inequi- 
lateral; umbones distant, recurved; siphonal region much the larger, biangulated; striz 
numerous, granulated, and slightly imbricated. 
Length, 3 inches ; height, 1 inch. 
Localities. Bracklesham, Selsey; var. 8, Barton. 
France, Grignon, Courtagnon, Senlis, Valmondois (Des/.). 
The ligamental area in this species has a broad, flat, and deep depression, with chevron- 
formed lines, which when the valves are united form lozenge-shaped ornaments imme- 
diately beneath the umbo, one within the other, the smaller closely approaching the 
hinge-margin ; sometimes there is another set of the lozenge-shaped marks on the siphonal 
region, but these latter are more often only parallels to the radiating umbonal lines. In 
specimens from Barton this area is tinged with a dark-red colour by the remains of the 
ligamental connector. The hinge-margin is furnished with numerous small teeth, rather 
more strongly displayed in the young state than in the old. Most of the large specimens 
from Bracklesham have the outside much eroded and nearly smooth, like specimens of 
A. tetragona that have inhabited some crypt where the movements of the animal have 
abraded the otherwise striated surface of the shell. This species is particularly abundant 
at Bracklesham, where it attains to large proportions. In the full-grown shell from that 
locality there is generally a large sinus, the ventral margin being deeply indented; but in 
the younger shells from the same locality this is not so strongly marked, and in some of 
those from Barton the valves are quite closed; it is so, however, with various specimens 
of A. tetragona. ‘The shell called 4. diangula, from the Bordeaux and Touraine beds, is by 
M. Deshayes considered as not identical with the Paris Basin species, and in this I think 
he is correct. There is, however, a slight difference between our own shell and that of the 
' Paris Basin, ours being more finely striated all over, especially so upon the pedal region. 
Mr. Sowerby has retained the name of 4. Branderi for the British fossil, which, he 
says (p. 169), ‘differs from 4. diangula, Desh., in having a much less acute keel.” The 
keel, however, is variable in that respect among the French fossils, and this difference does 
not always exist in the British shell. The pedal region in specimens from Barton is some- 
times broader or higher than in those from Bracklesham ; this arises from a less sinuated 
margin, which otherwise contracts the shell at that part; the large rays have generally a 
smaller intermediate one, and these are decussated by very visible lines of growth. The 
Barton shell may, perhaps, be considered as a variety under the name Branderi, but I think 
there is not a specific difference ; it much resembles 4. miniata, Desh. 
