FROM WEST PAKISTAN 39 



ridge and in the less evident anterior marginal rim. Further, the subcentral- 

 tubercle in Costa lies more towards the anterior. Hermanites has only two longitu- 

 dinal ridges and has a concave posterodorsal slope. Gyrocythere lacks the very wide 

 duplicature seen in Paracytheretta. 



Gyrocythere exaggerata sp. nov. 

 (Plate 18, figs. 10-14 »' Plate 19, Plate 20, fig. 5) 



Derivation of name. Latin exaggerates, exaggerated ; with reference to the well- 

 developed longitudinal ridges. 



Diagnosis. A species of the genus Gyrocythere with prominent eye-tubercle, 

 bilobate subcentral-tubercle and well-developed longitudinal ridges. 



Holotype. Io. 4331, a female carapace (PI. 19, figs. 1-4). 



Paratypes. Io. 4280 + 3122-3128 



Material. 39 specimens from the Zao River section from six horizons (sample 

 nos. 24145, 24147, 24148, 24150 24152). Eight specimens from the Rakhi Nala 

 section from two horizons (sample nos. 3613 and 3614) . GSP BM 2541-2. 



Type locality. Zao River section. 



Type horizon. Upper Chocolate Clays, sample no. 24151. 



Description. Carapace subrectangular in the male dimorph and subquadrate in 

 the female. Anterior margin broadly and evenly rounded, posterior narrow, almost 

 rounded in left valve but slightly subangular in the right valve. Dorsal and ventral 

 margins almost concealed in lateral outline by the dorsal and ventral ridges. Great- 

 est height in the ocular region, greatest length passes through the mid-point. Anterior 

 cardinal angle prominent with a concavity behind in lateral view. Valves almost 

 equal. Eye-tubercle prominent, rounded and polished, stands out in lateral view. 

 Subcentral-tubercle distinct and bilobate. Surface coarsely reticulate. Reticulae 

 are slightly papillose in some specimens. There are four well-developed longitudinal 

 ridges ; the dorsal ridge begins below the eye-tubercle anteriorly and is convex up- 

 wards, whilst the median or second ridge runs from the subcentral-tubercle pos- 

 teriorly and is convex upwards, its extension anterior to the subcentral-tubercle is 

 less well-marked. A third ridge is intercalated between the median and the ventral 

 ridges. It slopes obliquely upwards towards the posterior and is convex downwards 

 in its anterior part. The ventral ridge is restricted to the posterior two-thirds of the 

 carapace. It ends in a slight alar expansion in the posterior third of the carapace. 

 Anterior margin denticulate, posteroventral margin with short spines present in some 

 specimens. Normal pore canals simple, numerous (PI. 19, figs. 6, 7). Radial pore 

 canals not very well-displayed due to the form of preservation, but appear to be 

 simple, almost straight, irregularly spaced (few seem to bifurcate), with some 25 at 

 the anterior margin. Line of concrescence and inner margin coincide. Duplicature 

 of moderate width, 0-07 mm. anteriorly. Selvage pronounced in both valves ; it is in 

 the outer third of the duplicature in right valve but submarginal in left valve. Right 

 valve with well-developed7?ange groove, particularly on the venter. Muscle scars are 



