FROM WEST PAKISTAN 71 



Holotype. Io. 4355, a female carapace (Plate 35, figs. 2, 5, 6 ; Plate 36, fig. 2). 



Paratypes. Io. 4303 + Io. 3147 + Io. 3148 + Io. 3149. 



Material. Over 1400 specimens from the Rakhi Nala section from 17 horizons 

 (sample nos. 3448, 3451 to 3454, 3456, 3457, 3460 to 3467, 3470 and 3473). 470 

 specimens from the Zao River section from two horizons (sample nos. 24107 and 

 241 10). Approximately 600 specimens from the Shpalai Khwara section from three 

 horizons (sample nos. 24681, 24683 and 24686). GSP BM 2587-8. 



Type locality. Rakhi Nala section. 



Type horizon. Shales with Alabaster, sample no. 24173. 



Description. Carapace subrectangular in lateral view. Sexual dimorphism 

 rather pronounced ; the females are shorter, higher and wider than the males. 

 Anterior margin broadly and obliquely rounded, posterior nearly straight. Dorsal 

 margin almost intricate because of the over-reaching of the dorsal ridge, venter 

 slightly incurved in front of the middle. Valves almost equal except that the left 

 valve over-reaches the right in the region of the anterior cardinal angle and along the 

 posterodorsal corner. Eye-tubercle and subcentral-tubercle distinct. Shell surface 

 strongly reticulate. Three longitudinal ridges present ; the dorsal ridge starts 

 almost above the subcentral node and is arched convexly upwards ; the median ridge 

 commences from the eye-tubercle, curves sharply round to join anterodorsal corner 

 of the subcentral complex and continues posteriorly ; the ventral ridge slopes 

 obliquely upwards towards the posterior. The reticulations in the mid-posterior 

 region of the carapace show a concentric pattern, although it is not always present. 

 A high anterior marginal rim commences from the eye-tubercle continuing as a less 

 high rim round the venter and posterior. Anterior and posterior short marginal 

 spines present, 18-20 anteriorly. Normal pore canals simple, some 60 in the female 

 left valve. 24-25 anterior radial pore canals, simple straight, irregularly spaced, few 

 crossing one another, mostly terminating in marginal spines. Inner-margin and line 

 of concrescence coincide. Anterior duplicature 0-050 mm. wide in the female left valve. 

 Selvage strong in both valves, subperipheral in left valve but in right valve it lies 

 in the outer third of the duplicature. Right valve with deep anterior and ventral 

 flange grooves. Muscle scar pattern consists of four adductor scars in a vertical row 

 with an oval frontal scar. Hinge holamphidont, the details are as follows : 



Element 



Left valve 



Right valve 



Anterior 



Socket confluent with 



Strongly projecting sub- 





ocular sinus. 



conical or subpessular tooth 

 with a tendency for the 

 anterior profile in dorsal 

 view to appear concave. 

 The ocular sinus lies 

 distally beyond this and 

 opens to the interior below 

 and in front of it. 



