H 



w 



o-49 



o-49 



o-49 



— 



o-47 



— 



FROM WEST PAKISTAN 43 



cardinal angles well-marked, particularly in left valve. Left valve slightly over- 

 reaches the right valve in the anterodorsal corner and at the posterodorsal slope. 

 Greatest width in dorsal or ventral view lies in the posterior third. Subcentral- 

 tubercle prominent, eye-tubercle rounded and distinct. Surface strongly reticulate 

 with three longitudinal ridges : the dorsal ridge is curved convexly upwards in the 

 middle and starts anteriorly below the eye-tubercle ; the median ridge is less well- 

 developed, almost ill-defined in most specimens ; it runs posteriorly from the sub- 

 central-tubercle and is curved convexly upwards. The ventral ridge commences 

 anteriorly above the anteroventral corner and runs obliquely upwards towards the 

 posterior and culminates in the posterior third. Marginal rim is distinct at the 

 anterior and posterior but less distinct along the venter. Anterior margin denti- 

 culate, posterior extremity and posteroventral margin ornamented with about six 

 short spines or papillae. Duplicature of moderate width. Selvage well-developed, 

 subperipheral in the left valve but situated at some distance from the outer margin 

 in the right valve. The right valve has a fairly deep flange groove along the venter 

 and around the anterior margin. Hinge as for the genus. 



Dimensions (mm). 



L 



I°- 4333 Carapace male (holotype) o-88 



Io. 4283 Carapace female 0-83 



Io. 3119 Left valve female o-8o 



Comparison. Gyrocythere grandilaevis sp. nov. is somewhat similar to the present 



species and might even be ancestral although G. grandilaevis is smaller and has a less 



well-developed subcentral-tubercle. The dorsal and ventral ridges in G. grandilaevis 



are also less well-marked. G. mitigata differs from Gyrocythere exaggerata sp. nov. in 



being larger, having a different lateral outline and less emphatic ornamentation. 



Further, G. mitigata has three, rather than four longitudinal ridges and lacks a 



bilobate subcentral-tubercle. 



Remarks. G. mitigata has so far only been found in the Zao River section, where 

 it occurs at two horizons. 



Gyrocythere perfecta sp. nov. 



(Plate 22, figs. 1-10) 



Derivation of name. Latin perfectus, perfect ; with reference to the beauty of 

 the material. 



Diagnosis. Gyrocythere with strongly reticulate, concentrically arranged orna- 

 mentation. Eye-tubercle, subcentral-tubercle and longitudinal ridges distinct. 



Holotype. Io. 4335, a female carapace (PI 22, figs. 3, 4, 7, 8). 

 Paratypes. Io. 4284 + Io. 3120-1. 



Material. 20 specimens from the locality below from two horizons (sample 

 nos. 3498 and 3499). GSP BM 2548-50. 



Type locality. Rakhi Nala section. 



Type horizon. Lower Chocolate Clays, sample no. 3499. 



