649 



The dentition and general characters of this genus are the same 

 as those of Ct/theraa ; but the pecuhar flatness of the umbones, 

 the total want of sinus in the pallial impression, and the diverg- 

 ing tendency in the radiating stria? or ribs, together, seem to 

 separate this group from the rest of the Veneridce. It must be 

 observed, however, that these distinctions are not absolute. The 

 flatness of the umbones is not apparent in all the specimens of all 

 the species ; the diverging strise or ribs are not visible in the little 

 shell (wliich, nevertheless, we can find no place for anywhere else) 

 named C. minma ; and, with regard to the want of sinus in the 

 pallial impression, universal in this genus, the same character is 

 observed in numerous species of Cythercea and Venus ; and not 

 only so, but the passage from species with a distinct sinus to 

 those without any is so gradual, that it would be impossible to 

 rest a generic division upon that ground alone. The characters 

 must be taken, with all their exceptions, in combination, and 

 these shells, placed apart from the Cytlieroia, in the same case, 

 appeal to the eye as presenting a very distinguishable group. 



Sect. I. Testa ovalis, radiatim costata, varie fusco-rubescente 

 angulatim picta. 



1. Circe pectinata (pi. cxxxvii. f. 1, 2, 3). Venus pecti- 

 nata, Linn. Syst. Nat. p. 1135. 



Cir. testa oblonga, plerumque subcompressa, postice sub- 

 quadrata; costis nodulosis numerosis, postice divaricatis, turn 

 gradatim radiantibus, antice divaricatis; margine dorsah exca- 

 vate ; variat costis magnis. 



The ribs are numerous at the back, very diverging and bifui-- 

 cated, then at the angle gradually becoming more entire and 

 radiating, and at the front side very diverging again ; the dorsal 

 margin is decidedly excavated. Var. fig. 3 has the ribs as coarse 

 as in the next species, but the long ones near the posterior 

 angle do not quite reach the apex. 



Indian Ocean ; Lamarck. PhiKppines ; Cuming. Also, Red Sea. 



2. CmcE GiBBiA (pi. cxxxvii. f. 4, 5, 6, 7). Cythersea gibba, 

 Lamarck, no. 64. C. ranella, Lamarck, no. 65, inclusa. 



Cir. testa subovali, altiore, setate ventricosa, postice oblique 

 quadrata.; area distincta, subtrigona, divaricatim costata, tum 

 costis noduliferis duabus aut tribus magnis radiantibus, ad umbones 

 extensis ; medio et antice costis noduliferis, magnis, bifurcatis. 



4 p 



