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. 



Spondylus rarispina, Deshayes. Tab. VIII, fig. I, a, b. 



Spondylus rarispina. Desk. Coq. Foss. des Env. de Par., p. 321, pi. 46, figs. 6 — 10. 



— — Id. An. sans Vert, du Bassin. de Par., t. ii, p. 90, 1860. 



— — Nyst. Coq. Foss. Belg., p. 308, pi. 25, fig. 4 b, 1843. 



D'Orbiyny. Prod, de Paleont., t. ii, p. 393, No. 1116, 1850. 



— — Morris. Cat. Brit. Foss., p. 182, 1854. 



— — Pratt. Proc. Geol. Soc, vol. iv, p. 158, 1843. 



— If Archiac. Tr. Geol. Soc. Fr., 2d ser., t. iii, pt. ii, p. 437, 1850. 



— — J. Sow. in Dixon's Geol. of Suss., p. 173, 1850. 



— — Bellardi. Mem. de la Soc. Geol. de Fr., 2d ser., t. iii, p. 260, 



1852. 



— — Id. Catal. ragionato dei foss. Numm. d'Egitto, p. 26, No. 66, 



1854. 



Spec. Char. Sp. testa ovato-rotundatd, obliqud, 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 

 strioe or riblets, the larger having distant spines or imbrications. 



Longest diameter, \\ inch. 



Localities. Bracklesham {Edwards and Dixon). 



Belgium, les sables d'Uccle, de St. Gilles, et de Dieghem {Nyst). 

 France, a Chaumont {Deshayes), Biaritz {D'Jrch.), Nizza {Bellardi). 



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 



