FROM WEST PAKISTAN 



79 



Material. 42 specimens from the Rakhi Nala section from 5 horizons (sample 

 nos. 3610, 3613 to 3615 and 3617). 7 specimens from the Zao River section from 

 two horizons (sample nos. 24150 and 24152). GSP BM 2597. 



Type locality. Rakhi Nala section. 



Type Horizon. Upper Chocolate Clays, sample no. 3613. 



Description. Carapace subrectangular to subquadrate in lateral outline. 

 Sexual dimorphism moderate ; the presumed males are longer and less high than the 

 females. Dorsal and ventral margins straight and tapering towards the posterior. 

 Anterior margin broadly rounded, posterodorsal slope very slightly concave, posterior 

 extremity rounded, posterodorsal margin somewhat rounded. Anterior, posterior 

 and ventral margins decorated with a double row of short spines, but dorsal margin 

 with only one row of very short spines (these in some specimens almost look like 

 pustules). Greatest height at the anterior cardinal angle (which is obtuse and 

 angular) and greatest length through the mid-point. In dorsal view the greatest 

 width lies at the anterior node of the subcentral-tubercle. Valves almost equal. 

 Eye-tubercle rounded and distinct. Subcentral-tubercle divided into two nodes, 

 horizontally arranged, the anterior one being larger (spinose in some specimens). 

 The posterodorsal process consists of two nodes (spines in some specimens), which 

 are vertically disposed. In a few specimens a posteroventral node is also present. 

 Surface ornamented with scattered tubercles and spines. Duplicative fairly wide. 

 The selvage is subperipheral and well-developed in both valves. Radial pore canals 

 not seen because of mineralization. The adductor muscle scars are in an oblique row 

 of four at the posterior margin of the subcentral pit. The frontal scar is large and 

 U-shaped and opens towards the anterodorsal corner. Hinge holamphidont with 

 the following details : — 



Element 



Anterior 



Anteromedian 



Posteromedian 



Posterior 



Dimensions (mm). 



Left valve 

 Socket. 



Subcorneal tooth. 

 Denticulate ridge. 

 Fairly deep socket. 



Right valve 



Projecting subcorneal tooth. 



Socket. 



Locellate groove. 



Tooth, subpessular in dorsal 



view. 



Io. 4363 Carapace male (holotype) 



Io. 3159 Carapace female 



Io. 3158 Left valve male (broken) 



Io. 3156 Left valve female 



I°- 3 I 55 Right valve female 



Io. 3160 Carapace male 



Io. 3157 Carapace female 



Comparison. This species can easily be distinguished from Trachyleberis (Trachy- 

 leberis) lobuculus sp. nov. by its smaller size, slightly concave posterodorsal margin 

 and split subcentral-tubercle. Further, Trachyleberis (Trachyleberis) bimammillata 



L 



H 



w 



0-52 



0-29 



0-22 



0-50 



0-29 



0-24 



— 



0-29 



— 



0-50 



0-29 



— 



0-49 



0-29 



— 



o-54 



0-29 



0-22 



0-49 



0-29 



0-24 



