FROM WEST PAKISTAN 



55 



Type locality. Zao River section. 



Type horizon. Upper Chocolate Clays, sample no. 24173. 



Description. Carapace subrectangular in side view with dorsal and ventral 

 margins almost straight, subparallel ; anterior broadly rounded, posterodorsal margin 

 very slightly concave (particularly in the right valve), posterior extremity and 

 posteroventral margin rounded. Anterior cardinal angle well-developed in both 

 valves ; posterior cardinal angle in the left valve obtuse and well-marked but in the 

 right valve it is not very well developed. Left valve over-reaches the right valve 

 slightly at the anterior cardinal angle and at the posterodorsal slope, otherwise 

 the two valves are equal in size. In the dorsal view the greatest width passes 

 either through the subcentral-tubercle or through the posterior third with a 

 sulcus in between. Eye-tubercle rounded, polished and distinct ; subcentral-tubercle 

 prominent, composed of reticulate complex. Shell surface strongly reticulate with 

 three longitudinal ridges : the ventral ridge slopes obliquely upward towards the 

 posterior, ending abruptly just before reaching the compressed posterior platform; 

 the median ridge springs from the subcentral node, stretching backward to join the 

 ventral ridge and forms a posterodorsal loop. (In some specimens at certain 

 horizons longitudinal ridges are not well-developed.) A marginal rim runs round the 

 anterior, ventral and posterior margins — it is upraised around the anterior margin. 

 Anterior margin ornamented with approximately 20 small spines, posterior margin 

 with 6-8 spines. Normal fore canals fairly numerous (these become exaggerated in 

 specimens cleaned in ultrasonic vibrator). Radial pore canals simple, nearly 

 straight, irregularly spaced ; few cross one another. There are approximately 40 

 anterior radial pore canals and about 20 posterior canals. Inner margin and line of 

 concrescence coincide. Duplicature of moderate width (0-073 mm. at anterior, 

 0-06 mm. at posterior extremity). Selvage prominent, submarginal in left valve, 

 at some distance from the margin in the right valve. Flange groove well-developed 

 (particularly in the right valve) on the venter and around the anterior margin. 

 Adductor muscle scars are in a vertical column of four elongate scars, the second from 

 the top being longest and the bottom one the shortest. There are two frontal scars, 

 the top one is smaller and almost rounded, but the bottom one is ovate. Hinge 

 holamphidont : 



Element 



Left valve 



Right valve 



Anterior 



Deep socket bounded on 



Strongly projecting 





venter, confluent with 



slightly stirpate tooth 





ocular sinus. 



with a concavity on 

 anterior in dorsal view. 

 Ocular sinus opens below 

 and slightly anterior to it. 



Anteromedian 



Conical tooth with 

 straight anterior and 

 convex posterior in 

 dorsal outline. 



Deep rounded socket. 



