316 CRETACEOUS PELECYPODA 



notata, extra in facie laterali Icevigata et costa elevata ah apice ad marginem infero- 

 posteriorem decurrente marginata. 



The anterior straight or slightly flexuous termination of the ribs 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. 



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



Formation. — Arrialoor group. 



c. Group, — Gtahrm. 

 4. TuiGONiA ORiENTALis, Foi^hcs, PI. XV, Eigs. 16-17. 



1846. Trigonia orientalis et suhorhicularis, Forbes, Trans. Geol. Soe., Lond,, vii, p. 150, pi. xviii, figs. 11 and 10, 



1847. „ semiornata, d'Orb,, Yoy. Astrol., Paleont., pi. iv, figs. 31-32. 



1850. „ orientalis et suhorhicularis, Forbes, apud d'Orbigny, Prod. II, p. 240 ; — ecedem auctoriim. 



T. testa quadrangulariter suh-trigonata, moderate tumida, incequilaterali^ antice 

 hreviori, late et uniforme convexa, postice moderate producta^ regione ab umbone 

 ad marginem infero-posteriorem extensa obtuse angulata^ area super a ab eo angulo 

 rapide declivi, ad terminationem oblique sed late truncata, et sulco mediano indistincto 

 divisa, parte areali interiori concentrice multistriata, exteriori precipue prope marginem 

 posteriorem Icevigata ; superficie cetera costis numerosis concentricis, Icevibus, cequi" 

 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 " Glabr(B'' group, but at the same time not developed to a distinct rib, as 

 in the ^' CostatcB^ This area is divided by a very shallow sulcus in two sub-equal 

 halves : on the interior portion the concentric striae are distinct 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 shown 

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

 in the other, I do not see any essential difference between both. 



Locality, — ComarapoUiam and near Pondicherry, in a light grey and bluish 

 sandstone. 



Formation. — Arrialoor group. 



