OF SOUTHERN IXDIA, 353 



interno levi ; dentihiis cardinaUbns anticis muUo longioribus nuifjisque numerosis qticnn 

 posticis. 



Height of shell : its greatest length ... ... ... 0'88 



Thickness „ : „ „ ... ... ... 0-77 



Length of the largest specimen ... ... ... ... 140 m. m. 



The sub-9equilateral obliquely trigonal shape of the shell with its posterior 

 very prominent ridge, from which it rapidly slopes down, is a most distinctive 

 character of this species. The area is always very broad and angularly sulcated, 

 but its length varies, being anteriorly more or less produced ; posteriorly it is 

 always shorter. The hinge-teeth are more elongated anteriorly than posteriorly, 

 and in the middle they often are very small and irregular. The surface of the 

 shell is radiately and concentrically striated, the radiating striae are the stronger, 

 and the concentric ones are usually intersected at moderate distances by some- 

 what deeper furrows. In very large specimens the striation often entirely dis- 

 appears. 



Locality. — Near Serdamungalum, in a brownish limestone or whitish sand- 

 stone; not common. Forbes quotes the species from Verdachellum. 



Formation. — Trichiuopoly group. 



3. Trigonoauca Trichinopolttensis, Forbes, PI. XVIII, Figs. 12 and 14, 

 PL XIX, Figs. 2, 3, PI. XX, Figs. 2, 8, 9, 10. 



1846. Area TrichinopoUtensis, Forh., Trans. Geol. Soc, Lond., vii, p. 150, pi. xv, fig. 16 ; — eadem auctorum. 



Trig, testa elongate quadrangulari seu trapezoidali, obliqua, moderate tumida, 

 solidula, anlice ad marginem rotundata, postice ab iimboiiibus valde carinata, ter- 

 minatione inferiori prodncta, ad marginem oblique trimcata, striis radiantibus et 

 alteris decussantihus minutis, confertissimis, tecta ; area ligamentali moderate lata, 

 profunda, angulatim crasse sulcata ; cardo dentibus 5 anterior ibiis, et 5-6 posteriori- 

 bus instructus, primis interdum fere parallelis, alteris distinctim obliquis, medianis 

 parvulis, numerosis ; margine interno levigato. 



Height of shell : its length ... ... ,., 0'67 - 0'82 



Thickness „ : „ ... ... ... 0-54 - 0-65 



This is, as regards form, a very variable shell, some specimens being almost 

 quadrangular and others nearly trapezoidal, but they are posteriorly always strongly 

 ridged, considerably more produced than anteriorly, and at the margin very 

 obliquely truncate. The striation is peculiar and always traceable, except on the 

 posterior declivity, where the striae often become obsolete. The area is always 

 of moderate width, never as wide as in T. abnipta, and not so narrow as in Tr. 

 Telugensis. The hinge-teeth also differ from both species ; the anterior are some- 

 what bent outward and become almost horizontal and parallel among each other, 

 approaching in position those of the two species noted as Macrodon ( ? Gramma- 

 todonj, but the posterior teeth are regularly oblique, as in typical Trigonoarca. 



Localities. — Anapavidy, Garudamungalum, Serdamungalum, Alundanapooram ; 

 mostly in calcareous sandstone. 



Formation. — Trichiuopoly group. 



