OP SOUTHEEN INDIA. 459 



2. ExoGYRA OSTRACINA, Lamarck, PI. XXXV, Pigs. 6—12, and PL XXXVI, 



Pigs. 1 — 4. 



1801. Planospirites ostracina, Lamk., An. s. vert., p. 700. 



1820. Ostracites haliotoideus, Sclilotlieim, Petrefactenk., p. 28 (non Sow.). 



1821. „ auricularis, Wahlenberg, Petref., p. 58, (non Brong., 1822). 



1846. Exogyra stomatoidea,^ Forbes, Trans. Geol. Soc, Lond., vii, p. 156. 



1847. Ostrea crepidula, d'Orbigny, Yoy. d'Astrolabe, Paleont., pi. v, figs. 43-44 (non Desh.). 

 1850. „ suhinjlata, di'OxVignj,VYodL., 11,-^.2^0,, ex parte. 



1869. „ auricularis, Geinitz et 0. cornu-arietis, Coquand, Mon. Ostr. cret., p. 28, pi. viii, figs. 1 — 12. 



JEx, testa elongato ovata ; valva majore^ aut inferiore, plus mimisve elevata^ 

 nonnunquam planulata, props marginem superiorem angulata^ umbone valde incurvo 

 et spiraliter torto, striis et plicis incrementi concentricis et in declivitate super a ^ 

 plus minusve prcecipitata, nonnunquam costis nonnullis ohliquis notata, prope umhonem 

 sessili ; valva minor e, (aut super a J rare planata, scBpissime ad marginem convexum 

 elevata, in declivitate concentrice rugose striata et decussatim striolata, medio plana 

 aut concava, tenuis umbone valde intorta; sulco ligamentali angusto^ marginalia cur^ 

 vato, in valva majore costa elevata, in minore dente elongato^ ohttiso, infra marginato; 

 impressione musculari latiuscula, irregulariter suhquadrata^ excentrica, vmpressa^ 

 modo infra umhonem oriente ; margine palliali undique minute crenulato et striolato. 



This is an extremely variable species, but always possessing a more or less ovate 

 shape, with strongly spirally twisted beaks, usually attached by a portion of the 

 larger, or lower, valve. This latter is on the surface flattened, near the convex mar- 

 gin angulated, and from there more or less rapidly sloping towards the margin itself. 

 Young, or smaller, shells have this slope generally narrow, covered with the usual 

 concentric striae and plications of growth, or there are only a few oblique in- 

 distinct ribs traceable on the slope (see pi. xxxv, fig. 10), or the ribs are all of 

 considerable strength, as seen in fig. 8, pi. xxxv, and fig. 2, pi. xxxvi. The 

 gradations from one form to the other, collected at the same locality, are so gradual 

 that no specific importance can be attached to them. The plications are, however, 

 very different from those of Ex. plicifera^ Dnj., (Ostrea eadem apud Coquand). 

 In this species the ribs do not begin at the angle and extend over the entire slope, 

 but they originate below it, and are also much more numerous. 



Again, in larger shells the upper, or dorsal, slope often somewhat expands 

 posteriorly, and is then less precipitous (see pi. xxxvi, fig. 4). The strength and 

 reo'ularity of all the striae and folds of growth greatly depend upon tlie regularity 

 of the object on which the species is sessile. This, partially at least, also applies to 

 the strength of the ribs on the upper slope ; for these ribs are, as a rule, only 

 found developed in such specimens as possess an irregular place of attachment. 

 When the upper somewhat perlaceous layer of the shell is decomposed, the inner 

 layers exhibit a fine radiating striation, apparently produced by the tubular struc- 

 ture of the shell (see pi. xxxvi, figs, ^a & 45). 



The smaller or upper valve is rarely flat,, more generally concave, along the 

 convex margin considerably raised and marked with concentric, close plications and 



* Errore-tyjDico staiimaloidea apud d'Orbigny, Prod, ii, p. 256, et staumatoidea apud Coquand. 



