OF SOUTHERN INDIA. 311 



area very large, truusversally striated or finely costated, laterally well margined ; 

 sides of shell ornamented with strong transversally curved and more or less strongly 

 tuberculated ribs, which in front become more or less irregular and sometimes 

 partially obsolete. The best known species of this type is probably T. scabra, 

 Lam. This group is equally represented in Jurassic as in cretaceous strata. 



b. JJndulatce. Shell moderately produced posteriorly, with a rapidly descend- 

 ing areal margin ; area transversally striated, not distinctly margined laterally ; 

 sides ornamented with angularly bent ribs, the longer anterior portions of them 

 being arranged concentrically, the posterior more or less obliquely descending or 

 even quite vertical ; the ribs are generally partially smooth, partially tuberculated ; 

 type, Tr. sulcataria, Lam. This group is rejiresented in the Jurassic and ci-eta- 

 ceous strata. A peculiar form mostly partaking of the characters of this group is 

 d'Orbigny's Tt\ Hanetiana from the uppermost cretaceous beds of Chili. The 

 ribs are in front almost vertical and towards the area scarcely angular. 



c. Costatce. Shell sub-trigonal, high, beaks sharp and prominent, posteriorly 

 more or less sharjily descending, area broad, margined laterally by a strong rib and 

 usually ornamented with granulated decussating striae of which the radiating ones 

 predominate ; sides ornamented with concentric smooth ribs ; type, Tr. costata. 

 Lam. The Costatce are chiefly Jurassic species, only few occur in cretaceous 

 deposits. 



d. GlabrcB. Shell sub-quadrangularly or elongately ovate, moderately com- 

 pressed ; area laterally not margined, or only indicated by an indistinct groove ; 

 sides ornamented with smooth concentric ribs, sometimes extending on the areal 

 region and at other times becoming quite obsolete before they reach it ; tyjie, 

 Tr. longa, Ag. This group is very rare in the Jura, and most of the known 

 species are cretaceous. 



e. Qttadrat(B. Form sub-quadrangular, broadly truncate behind ; area indis- 

 tinctly margined ; sides ornamented with irregular divaricating and bifurcating 

 sub-obsolete ribs, frequently broken into numerous granules or tubercles; type, 

 Tr. dcedalcea. Park. The Qiiadratce are scarce, but occur in both the Jurassic and 

 cretaceous deposits. 



f. PectinatcB. Shell broadly oval, sub-caudate posteriorly, area not distinct; 

 the whole shell ornamented with radiating sub-tuberculated ribs ; type, Tr. marga- 

 ritacea, Lam. ; this group is only found recent. 



All the groups with the exception of the last are found represented in cre- 

 taceous deposits, as well as in the jurassics. Giebel mentions that the genus also 

 occurs in Devonian rocks, but I have not seen any typical species from beds older 

 than the Lias. Tertiary species are only very few. 



Giebel described (Jahresb. Ver., Halle, 1852, v,p. 379, pi. v, fig. 2), a Tr. sep- 

 taria from tertiary beds near Biere ; it belongs to the Scaplioidea group, and some- 

 what resembles the South American Tr. Hanetiana, d'Orb., first noted from 

 a tertiary deposit, but subsequently transferred l)y the same author into the 



Senonien. 



4g 



