or SOUTHERN INDIA. 459 



2. ExoGYRA OSTRACINA, Lamcirck, PI. XXXV, Figs. 6—12, and PL XXXVI, 



Figs. 1 — 4. 



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



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



-^^21. „ auricularis, Wahlenberg, Peh-ef., p. 58, (non Brong., 1822). 



18-16. Exogyra stomatoidea * Forbes, Ti-ans. Geol. Soc, Lond., vii, p. 156. 



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



1850. „ subinflata, d'Orbigny, Prod., ii, p. 256, ex parte. 



1869. „ auricularis, Geinitz et O. cornu-arielis, Coquaud, Mon. Ostr. cret., p. 28, pi. viii, figs. 1—12. 



Ex. testa eloiigato ovata; vcdva majore, ant infer lore, plus mimisve elevata, 

 nonnunquam plamdata, prope marginem superlorem angulata, timhone valde iiicurco 

 et spiraUter torto, striis et pUcis incrementi concentncls et in declivitate siqjera, 

 plus minusve prcBcipitata, nonmmquam costis nonnulUs obliquis notata, prope nmbonem 

 sessUi; valva miiiore, faut supera,) rare planata, scepissime ad marginem convexum 

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

 ant coHcava, tenui, tiinbone valde intorta ; sulco Ugamentali angusto, marginali, cur- 

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

 impressione musculari latiuscula, irregulariter subquadrata, excentrica, impressa, 

 modo infra umbonem oriente ; margine palliali undiqiie minute crenulato et striolata. 



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

 shape, with strongly spu-ally 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 angvdated, 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. plicfera, Duj., (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 

 regularity of all the striae arfd folds of growth greatly depend upon the 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. 4« & 46). 



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-typico staumaloidea apud d'Orbigny, Prod, ii, p. 256, et staumatoidea apud Coquand. 



