316 CRETACEOUS PELECYPODA 



notcita, extra in facie laterali Icevigafa et costa elevata ab apice ad marginem infero- 

 posteriorem clecurrcnte marffinata. 



The anterior straight or slightly flexuous termination of the rihs and the 

 large prolonged area readily distinguish this species from others ; it belongs to a 

 group chiefly Jurassic, and only rarely represented in cretaceous deposits. It seems 

 to be a very rare species. 



Localitif. — South-east of Arrialoor, in a whitish silicious sandstone. 



Formation, — Arrialoor grovip. 



c. Group, — Glabra;. 

 4. Trigonia orientalis, Forbes, PI. XV, Figs. lG-17. 



1846. Trigonia orientalis et suhorUcularis, Forbes, Trans. Geol. Soc, Lond., vii, p. 150, pi. .wiii, fig.s. 11 and 10. 



1847. „ se-miornata, d'Orb., Voy. Astrol., Pale'out., pi. iv, figs. 31-32. 



1850. „ orientalis et suhorhiculayis, Forbes, apud d'Orbiijny, Prud. II, p. 240 i—eaclem auctoruni. 



T. testa quadrangulariter suh-trigonata, moderate timiida, incBquilaterali, antice 

 breviori, late et imiforme convexa, postice moderate producta, regione ab umbone 

 ad marginem iufero-posteriorem extensa obtuse angiilata, area sttpera ab eo angtdo 

 ra^nde decUvl, ad termmalionem oblique sed late truncata, et sulco mediano indistincto 

 divisa, parte areali interiori concentrice multistrlata, exteriori precipue proj)e marginem 

 posteriorem IcBvigata ; superficie cetera costis numerosis concentricis, Icevibus, ceqiii- 

 distantibus, antice ad marginem fere obsoletis induta. 



The young shells generally are much flatter and always more distinctly sub- 

 quadrangular than old ones ; in the latter the posterior part of the shell becomes 

 more produced, while the anterior does not increase at the same ratio. The 

 concentric ribs are numerous, smooth, very gradually increasing in thickness as the 

 shell grows, becoming at the same time more or less distant from each other ; near 

 the periphery the furrows equal in width the thickness of the ribs ; they terminate 

 anteriorly with a very slight flexuosity, but become nearly quite obsolete before 

 they reach the margin. The area is very broad, separated from the costated portion 

 of the valves by an obtuse angle, not quite so indistinct as in the typical forms 

 of the " Glabra;" group, but at the same time not developed to a distinct rib, as 

 in the " Costata;." This area is divided by a very shallow sulcus in two sub-equal 

 halves : on the interior portion the concentric striae are distiuct in all stages of 

 growth, but on the outer portion they become more or less obsolete, especially 

 towards the margin. 



I have compared both of Eorbes' originals ; they are not so perfect as showji 

 in his figvires, but except that in the one the ribs are slightly more distant than 

 in the other, I do not see any essential difi'erence between both. 



Localitif. — Comarapolliam and near Poudicherry, in a light grey and bluish 

 sandstone. 



Formation. — Arrialoor group. 



