FROM WEST PAKISTAN 67 



Description. Sexual dimorphy rather apparent ; the females are shorter, higher 

 and wider than the males. Carapace subrectangular in side view with the greatest 

 height at the anterior cardinal angle and the greatest length below the middle. 

 Anterior margin broadly and evenly rounded, posterodorsal slope very slightly 

 concave, posterior extremity subtriangular, postero ventral margin almost straight. 

 Dorsal margin very slightly undulating, venter slightly incurved in front of the middle. 

 Posterior portion of carapace compressed. Anterior marginal rim high continuing 

 along the venter and around the posterior end as a somewhat less elevated rim. 

 Anterior margin set with numerous small and delicate denticles, posterior with a few 

 denticles. Anterior and posterior cardinal angles protruding. Left valve over- 

 reaches the right valve slightly at the anterior cardinal angle and posterodorsal 

 slope. In dorsal view the greatest width is situated almost in the middle of the 

 carapace. Eye-tubercle distinct. Subcentral-tubercle more or less distinct. Surface 

 reticulate with a ventral ridge, which slopes slightly upwards towards the posterior. 

 A short curved, hornlike ridge at the posterodorsal corner is present (better seen in 

 dorsal view). Internal details not known. 



Dimensions (mm). 



L H W 



Io. 4301 Carapace male 0-68 0-35 0-27 



Io. 4350 Carapace female (holotype) 0-63 0-37 0-29 



Comparison. Quadracythere (Hornibrookella) subquadra sp. nov. is subquadrate 

 in lateral outline, has deeper reticulations and a better developed subcentral-tubercle. 



Quadracythere (Hornibrookella) arcana (Lubimova and Guha) 



(Plate 34, figs. 3-5) 



i960 Cythereis arcanus (sic, recte Cythereis arcana) Lubimova and Guha, p. 33, pi. 3, fig. ia-b. 



Diagnosis. Carapace with distinct caudal process. Surface coarsely reticulate 

 with superimposed longitudinal lineations in posterior half of carapace. 



Figured specimen. Io. 3142. 



Material. Two specimens from the locality and horizon below. 



Locality. Rakhi Nala section. 



Horizon. Lower Chocolate Clays, sample no. 3499. 



Description. Carapace thick-shelled, subquadrate in lateral view. Anterior 

 margin broadly rounded, posterior with a pronounced caudal process. Dorsal margin 

 slightly concave behind the round and protruding anterior cardinal angle, particularly 

 in the right valve. Ventral margin almost straight. Greatest height at the anterior 

 cardinal angle and greatest length below the middle. Left valve slightly over- 

 reaches the right in the region of the anterior cardinal angle and posterodorsal slope. 

 Eye-tubercle rounded and distinct, subcentral-tubercle well-developed. Surface 

 ornamentation consists of coarse reticulations with superimposed longitudinal 

 lineations in the posterior half of the carapace. The ventral ridge is slightly concave 

 downwards culminating in an ala posteriorly. A short ridge at the posterodorsal 



