30 THE VOYAGE OF H.M.S. CHALLENGER. 



present form. On the contrary, the left or smaller valve of Corhula tunicata has only 

 the earlier portion strongly ridged, the rest of the surface exhibiting only lines of growth 

 coated with a fibrous epidermis. The contrast between the two styles of ornamentation 

 is so marked that the valve presents the appearance of having the umbonal portion of a 

 differently sculptured species fixed on to its own smoother surface. The peculiarity, in 

 the two specimens from the Arafura Sea, consists in their having a much larger proportion 

 than usual of this valve strongly sculptured. In one there is scarcely any cessation of 

 the concentric ridging, whilst in the other it extends over about two thirds of the surface. 

 The ridges in this species are both more numerous and more rounded than in Corhula 

 crassa, which also presents another distinction in very adult examples that is never met 

 with in any specimens of any age of Corhula tunicata. I refer to the presence of fine 

 denticulation upon the dorsal and ventral margins of the left valve. A still closer 

 approach to the present species is met with in Corhula sulcata from West Africa. 

 Here is a species which not only has the same form, but offers scarcely any difference in 

 sculpture, and is mainly distinguished by a variation in colour. 



Corhula crassa, Hinds, var. 



Corhula crassa, Hinds, Proc. Zool. Soc. Lond., 1843, p. 53. 

 Corhula crassa, Keeve, Conch. Icon., vol. ii. figs. 8 a-c. 



Habitat. — Torres Strait, in 3 to 11 fathoms (Challenger); Port Essington (J. B. 

 Jukes, Esq., in Brit. Mus.); Straits of Macassar, Malacca (Hinds) ; Bai.s, Island of 

 Negros, Philippines (Cuming). 



The specimens from the above localities are somewhat different from the normal form 

 found at the Philippine Islands. In stoutness and outline they correspond very closely, 

 but are at once distinguished by the microscopic sculpture. The entire surface of the 

 valves is covered with a dense mass of minute circular granules, arranged more or less 

 regularly in closely packed radiating series. In the typical form the rows of granules, 

 which are of the same character, are much farther apart, and the apices of the valves 

 are smoother than in the variet)^ which has the concentric ribs developed somewhat 

 earlier. Beyond these differences there does not appear an}^ reason for separating these 

 two forms. 



Corbula macgillivrayi, n. sp. (PI. X. figs. 8-8?>). 



Testa magna, elongata, crassa, albida, paulo insequivalvis, valde insequilateralis, antice 

 rotundata, postice oblique truncata, ad extremitatem acute angulata, concentrice costata, 

 et radiatim minute granulato-striatn, costis ])one carinam ab umbone ad extremitatem 



