BIVALVIA. 51 
expansion ; but in many it is bipartite, extending outwardly in a furrow, like the oyster ; a 
small ligament sometimes occupies the hinge-margin, and this margin is occasionally rugose 
or faintly denticulated. The lower valve is generally the most spiny, and in the recent 
state is nearly colourless ; this is the right valve, and the one by which it is most frequently 
attached. Many fossil species are known, commencing low in the Secondary series. The 
structure of the shell in some of the species has long been noticed as being composed of 
two different layers (and in the recent shells of two different colours), the inner being very 
distinct from the outer, and very destructible. 
This genus in the fossil state is supposed to indicate, for the formation in which it is 
found, somewhat of a tropical character; it may be so as a general rule, but three species 
are found at the present day, in the Mediterranean, in association with northern forms. 
It is purely a marine genus, and the living species are generally found in deep water. 
Sponpyus RARISPINA, Deshayes. ab. VIII, fig. 1, a, 4. 
SponpyLus RaRISPINA. Desh. Coq. Foss. des Envy. de Par., p. 321, pl. 46, figs. 6—10. 
— — Id. An. sans Vert. du Bassin. de Par., t. 11, p. 90, 1860. 
— — Jyst. Coq. Foss. Belg., p. 308, pl. 25, fig. 4 6, 1843. 
— — D Orbigny. Prod. de Paléont., t. ii, p. 393, No. 1116, 1850. 
— — Morris. Cat. Brit. Foss., p. 182, 1854. 
— = Pratt. Proc. Geol. Soe., vol. iv, p. 158, 1843. 
— --— D aArchiac. Tr. Geol. Soc. Fr., 2d ser., t. iti, pt. il, p. 437, 1850. 
— — J. Sow. in Dixon’s Geol. of Suss., p. 173, 1850. 
= — Bellardi. Mém. de Ja Soc. Géol. de Fr., 2d ser., t. i, p. 260, 
1852. 
— — Id. Catal. ragionato dei foss. Numm. d’Egitto, p. 26, No. 66, 
1834. 
Spec. Char. Sp. testd ovato-rotundata, obliqua, brevi-auritd ; sulcis vel costis radian- 
tibus, numerosis ; majoribus spinis, raris, echinatis, alteris subaqualibus muticis. 
Shell roundly ovate, oblique, with small auricles, covered with numerous radiating 
striae or riblets, the larger having distant spines or imbrications. 
Longest diameter, 1} inch. 
Localities. Bracklesham (Edwards and Divon). 
Belgium, les sables d’Uccle, de St. Gilles, et de Dieghem (Vysf). 
France, 8 Chaumont (Deshayes), Biaritz (D’ Arch.), Nizaa (Bellard:). 
I have seen only two specimens of this species, and those are both of the left or free 
valve. Both these specimens are alike oblique in form; the larger rays are sparingly 
covered with small spines or imbrications, and between these are three, sometimes two, 
intermediate rays, which are generally smooth. This valve is much depressed ; the right 
or adherent one was probably tumid. The hinge is strong, and in this valve there are 
