20 



EARLY TERTIARY OSTRACODA 



Paratypes. Io. 4266-9 + lo. 3107-12. 



Material. Over 800 specimens from the Zao River section from 20 horizons 

 (sample nos. 24131, 24155, 24157, 24159, 24170, 24173 to 24178, 24180, 24181, 24183 

 to 24188 and 24195). Approximately 300 specimens from the Rakhi Nala section 

 from 20 horizons (sample nos. 3624 to 3626, 3630, 3631, 3634, 3640 to 3642, 3645, 

 3646, 3648 to 3653, 3658 and 3660). GSP BM 2157-8. 



Type locality. Zao River section. 



Type horizon. Upper Chocolate Clays, sample no. 24155. 



Description. Sexual dimorphism rather marked, the males are more elongate 

 than the females. Carapace subrectangular in lateral view. Dorsal margin sinuous, 

 ventral margin straight or evenly curved. Anterior margin broadly rounded, 

 posterodorsal margin very slightly concave, posterior extremity rounded, postero- 

 ventral margin rounded or almost straight. Anterior and posterior cardinal angles 

 well-developed and protruding. Valves nearly equal. Eye-tubercle distinct, 

 polished and rounded. Subcentral-tubercle distinct. Surface either reticulate (with 

 or without superimposed papillae) or papillose. There are three posterior transverse 

 concentric ridges approximately parallel to the posterior margin with grooves in 

 between. There is a short ridge in the antero ventral area running obliquely from 

 the anterior towards the venter, with a groove on the dorsal side. Anterodorsal 

 groove fairly deep and bounded on the anterior by a short almost vertical ridge 

 running from the eye-tubercle. The marginal rim is high in the anterior but less high 

 on the venter and posterior. Anterior and posterior margins ornamented with short 

 spines, only present in some specimens and approximately 20 anteriorly and 10 

 posteriorly. Normal pore canals simple, small and numerous. Radial pore canals 

 numerous, simple, nearly straight, some in groups of two or three, often apparently 

 crossing one another. Duplicature moderately wide. Selvage pronounced in the 

 outer third of the duplicature in the right valve but sub-marginal in the left valve. 

 Right valve with deep ventral and anterior flange grooves. Line of concrescence and 

 inner margin coincide. Muscle scars consist of four adductors in a vertical row with 

 an oval frontal scar and two almost rounded mandibular scars below. Hinge 

 holamphidont : 

 Element 

 Anterior 





Anteromedian 



Posteromedian 

 Posterior 



Left valve 



Deep rounded socket 

 bounded on all sides, 

 confluent with ocular 

 sinus. 



Subcorneal projecting 

 tooth with a straight 

 anterior and convex 

 posterior in dorsal view. 

 Denticulate bar 

 Deep elongate socket 

 unbounded on venter 



Right valve 



Strongly projecting stirpate 



tooth (ocular sinus situated 



below and slightly anterior 



to it). 



Deep socket. 



Locellate groove 

 Pessular tooth with a 

 tendency towards reniform, 

 higher on posterior. 



