OF SOUTHERN INDIA. 325 



XXXV. Family— NUCULIB^. 



Animal oval, with the mantle margins entirely open, with ciliated edges, which 

 on the shorter, or posterior side, where the siphons should he placed, are a little longer 

 than anywhere else ; there are no distinct siphons present ; gills elongated, un- 

 equal ; lahial palps rather large, suh-triangular, unequal, the upper shorter, 

 thickened, and curled, the lower elongated and narrower ; foot geniculate, expansible 

 into a serrated disc, which can be folded together. 



The shells are oval, produced anteriorly, very short, and sub-truncate poste- 

 riorly ; hinge composed of two comb-like serrated or dentate hinge margins, ligament 

 internal in a special pit, pallial line entire; internal layer generally strongly 

 nacreous. 



The NucuLiDM differ from the last family by the total want of special siphons 

 and by the entire pallial impression ; the shells are also readily distinguished by being 

 very short posteriorly. Besides the recent species of Niicula, the shells of which are 

 always pearly inside, there has lately been a jieculiar shell made known under 

 the name of Sarepta ; it is of a more oval form (something like a short NiiculanaJ, 

 but not nacreous internally. The other characters are those common to Nucula, 

 and it seems to me, therefore, that Sarepta may more appropriately be classed 

 in this family than in the Nuculanid^. 



Eossil species of Nuculid^ occur in all formations, and their geological and 

 geographical distribution is world-wide. There are at least 50 species known from 

 the Palaeozoics, about 30 from the Trias ; the same nvimber barely increases in the 

 Jurassic time, which is rather singidar; 71 species are on record from cretaceous 

 deposits, and somewhat more than one hundred from the tertiaries, while there 

 are hardly more than about 40 recent species known. This decrease is remark- 

 able as compared with the Nuculanidje, in which we see an increase from the 

 tertiary into the present period. The species live on muddy ground, in the tropical 

 seas mostly between three and six fathoms, but are found at a much greater depth 

 in northern seas. 



a. Siih-famihj,—NUCVLIN.E. 



1. Nucula, Lam., 1799. Shell sub-trigonal or obliquely ovate, tumescent, 

 inequilateral, posterior side very short and sub-truncate, anterior much produced 

 and generally rounded at the end, surface usually smooth, covered Avith a thin 

 olivaceous epidermis, highly nacreous within ; hinge-line angulated and denticulated 

 on both sides of the cartilage pit, which lies below the beak and extends anteriorly ; 

 sometimes there is a thin sub-external ligament present; type, N. nucleus, Linn. 



la. Acila, H. and A. Adams, 1857, has been suggested for the divaricately 

 or angularly sculptm-ed species, of Avhich N. divaricata. Hinds, is the type. 



b. Sab-famili/,—SAREPTINjE. 



2. Sarepta, A. Ad., 1860, (Ann. & Mag. Nat. Hist., 3rd ser., V, p. 303). 

 Shell oval, equivalve, not pearly within ; hinge-line nearly straight, provided with 

 numerous denticles, cartilage internal below the beak ; muscular impressions 

 distant, pallial line entire; type, S. speciosa, A. Ad., from the Japan seas. "This 



