FROM WEST PAKISTAN 83 



broadly and evenly rounded, posterior slightly sub-triangular in right valve but 

 almost rounded in the left, posterodorsal margin very slightly concave. Anterior 

 and posterior cardinal angles well-developed. Sexual dimorphism rather strong ; 

 the presumed males are longer, less high and less wide than the females. Valves 

 almost equal. In dorsal view greatest width lies at the anterior third (in the region 

 of the subcentral-tubercle). Eye-tubercle rounded, polished and prominent (standing 

 out from the carapace). Subcentral-tubercle distinct. Surface ornamentation consists 

 of combination of reticulations and small spines. The posterodorsal process is 

 divided into two spines (although in some specimens this division is not detectable). 

 In a few specimens a posteromedian process is also developed. The anterior and 

 posterior margins are decorated with a double row of spines ; the second row lies on 

 high anterior and posterior marginal rims. The posterior marginal spines are 

 larger and less in number. Duplicature fairly wide. Selvage prominent and sub- 

 marginal. Radial pore canals not clearly displayed because of mineralization, but 

 appear to be simple, more or less straight with median swellings, 30-35 anteriorly. 

 Hinge holamphidont : 



Element Left valve Right valve 



Anterior Socket. Stirpate tooth. 



Anteromedian Subcorneal tooth. Deep socket. 



Posteromedian Locellate shallow groove. Denticulate bar. 



Posterior Deep socket. Tooth, almost rounded in 



lateral view. 



Dimensions (mm). 



L H w 



Io. 4361 Carapace male (holotype) 0-62 0-30 0-22 



Io. 3162 Carapace female 0-52 0-29 0-21 



Io. 3164 Right valve male o-6i 0-29 — 



Io. 3163 Right valve female 0-50 0-28 — 



Comparison. The present species shows some affinity to Trachyleberis (Acantho- 

 cythereis) decoris sp. nov. but is shorter, less high and less wide. These two species 

 also differ in surface ornamentation. T. (A.) postcornis has a combination of 

 reticulations and small spines, while T. (A.) decoris is reticulate with superimposed 

 pustules. Further, T. (A.) postcornis has a well-developed posterodorsal process 

 divided into two spines and a distinct subcentral-tubercle. This species may also be 

 distinguished from T. (A.) procapsus sp. nov. in being smaller and lacking a smooth 

 walled area behind the anterior marginal rim. These two species also differ in dorsal 

 outline. 



Trachyleberis (Acanthocythereis) decoris sp. nov. 



(Plate 42, figs. 3-6, 8, 9) 



Derivation of name. Latin decoris, beautiful, adorned ; with reference to the 

 bejewelled appearance of the pustules and reticulae. 



Diagnosis. Acanthocythereis in which surface ornamentation consists of reticula- 



