FROM WEST PAKISTAN 41 



Type locality. Rakhi Nala section. 



Type horizon. Green and Nodular Shales, sample no. 3407. 



Description. Sexual dimorphism distinct ; the males are more elongate than the 

 females. Carapace subrectangular in side view. Anterior margin broadly rounded, 

 posterior subtriangular. Dorsal margin straight but appears slightly convex in 

 lateral view due to the over-reaching of the dorsal ridge, ventral margin slightly 

 concave in front of the middle. Anterior cardinal angle distinct with a concavity 

 behind in lateral view. Left valve slightly over-reaches right valve at anterior 

 cardinal angle and posterodorsal slope. In dorsal view the greatest width lies in the 

 anterior two-fifths. Subcentral-tubercle well developed. Eye-tubercle distinct. Surface 

 strongly reticulate with three longitudinal ridges ; the dorsal ridge is curved con- 

 vexly upwards ; the median ridge is more or less ill-defined in many specimens ; the 

 third or ventral ridge is curved convexly downward anteriorly. Anterior and 

 posterior marginal rims present but not high. Both anterior and posterior margins 

 are denticulate. Duplicature of medium width. Selvage distinct and at some 

 distance from the outer margin in right valve. Anterior and ventral flange grooves 

 well-developed in right valve. Radial pore canals not clearly seen due to mineraliz- 

 ation. Hinge as for the genus. 



Dimensions (mm). 



L 

 Io. 4334 Carapace male (holotype) o-68 



Io. 4281 Carapace female 0-67 



Comparison. This species is smaller than Gyrocythere grandilaevis sp. nov. 

 Although the longitudinal ridges are no better developed than G. grandilaevis the 

 eye-tubercle and subcentral-tubercle are more prominent, the reticulation is deeper 

 and wider in proportion. 



Gyrocythere grandilaevis sp. nov. 

 (Plate 20, figs. 9-11, 13 ; Plate 21, figs. 1-4) 



Derivation of name. Latin grandis, large + laevis, smooth ; with reference to 

 the carapace. 



Diagnosis. A species of Gyrocythere with large, reticulate, smooth carapace. 

 Three longitudinal ridges, including median ridge which is not well-developed. 

 Anterior and posterior marginal rims distinct. 



Holotype. Io. 4332, a female carapace (PI. 20, figs. 11, 13 ; PI. 21, figs. 3, 4). 



Paratype. Io. 4282. 



Material. 16 specimens from the locality below from four horizons (sample 

 nos. 3463 to 3466). Two specimens from the Shpalai Khwara section from one 

 horizon (sample no. 24692). GSP BM 2545-6. 



Type locality. Rakhi Nala section. 



Type Horizon. Shales with Alabaster, sample no. 3463. 



H 



w 



o-37 



o-34 



0-42 



o-37 



