FROM WEST PAKISTAN 51 



out a dorsal ridge, ventral ridge well-marked, anteromedian swelling distinct, 

 posterodorsal tubercle present. 



Holotype. Io. 4341, a female carapace (PL 25, figs. 15) ; (PI. 26, figs. 2, 3). 



Paratypes. Io. 4289-93 + Io. 3137-40. 



Material. Approximately 800 specimens from the Rakhi Nala section from 42 

 horizons (sample nos, 3160, 3163, 3167, 3170, 3171, 3173, 3174, 3177, 3179, 3180, 

 3186-3194, 3197 to 3200, 3401 to 3405, 3407, 3409, 3410, 3415, 3418 to 3423, 3428, 

 3432, 3434 and 3435). GSP BM 2560-65. 



Type locality. Rakhi Nala section. 



Type horizon. Upper Rakhi Gaj Shales, sample no. 3167. 



Description. Sexual dimorphism rather pronounced, the males are longer in 

 proportion than the females. Carapace subrectangular or wedge-shaped in lateral 

 outline. Dorsal and ventral margins almost straight, tapering towards the posterior, 

 anterior margin broadly and evenly rounded, posterior narrow, slightly subangular 

 in the middle or almost rounded. Greatest length passes through the middle, 

 greatest height lies at the anterior cardinal angle which is fairly well-developed. Left 

 valve slightly over-reaches the right valve at the anterior cardinal angle and in the 

 region of the posterodorsal slope. Eye-tubercle rounded and distinct, lying on the 

 anterior marginal rim. Subcentral or anteromedian swelling distinct, either elongate 

 or almost rounded (elongate in most morphotypes) . There is no dorsal ridge. Ventral 

 ridge fairly well-developed, short in most morphotypes, either almost straight or runs 

 slightly obliquely towards the posterior end where it may be connected to the second 

 posterior tubercle or to the posterodorsal tubercle by means of a short vertical ridge. 

 A dorsal ridge runs between the second posterior tubercle and the anteromedian 

 swelling in some morphotypes. Anterior marginal rim high, ventral marginal rim 

 less distinct, posterior marginal rim distinct. Short spines decorate the anterior and 

 posterior margins. 



Trends of variants : — 



1. Become sparsely punctate. 



2. Gain second posterior tubercle, which may join either the posterodorsal 

 tubercle or the ventral ridge. 



3. Become wedge-shaped. 



Remarks. 0. peristicta commonly occurs in the Upper Rakhi Gaj Shales, Green 

 and Nodular Shales and Rubbly Limestones of the Rakhi Nala section. 

 This species may be divided into the following morphotypes : 



Morphotype A 



(PL 25, figs. 13-17 ; PL 25, figs. 1-3). 



This has a well-delimited vertical posterodorsal tubercle. There is no second 

 posterior tubercle. The anteromedian swelling is less well-developed. The ventral 



