FROM WEST PAKISTAN 29 



Diagnosis. Costa with a fourth ventral ridge intercalated between third ridge 

 and ventral margin. 



Type species. Costa (Paracosta) declivis sp. nov. 



Remarks. The subgenus Paracosta is so far only known from the Rakhi Nala 

 section. It is represented by two species in the Upper Chocolate Clays and one 

 species in the Pellatispira Beds. 



Costa (Paracosta) declivis sp. nov. 



(Plate 13, figs. 8, 10-14 ; Plate 14, figs. 1, 2) 



Derivation of name. Latin declivis, sloping downward ; referring to the 

 direction of the ridge running anteroventrally from the subcentral-tubercle. 



Diagnosis. A small species of Paracosta in which longitudinal ridges are well- 

 developed, median or second ridge runs anteroventrally from subcentral-tubercle. 



Holotype. Io. 4325, a male carapace (pi. 13, figs. 8, 10-12). 



Paratypes. Io. 4273 — Io. 3116. 



Material. 34 specimens from the Rakhi Nala section from four horizons (sample 

 nos. 3661 to 3664) . GSP BM 2526-7. 



Type locality. Rakhi Nala section. 



Type horizon. Pellatispira Beds, sample no. 3662. 



Description. Sexual dimorphism marked, the males are longer than the females. 

 Carapace elongate, subrectangular in lateral view with greatest height at anterior 

 cardinal angle. Dorsal and ventral margins almost straight, subparallel, anterior 

 end broadly rounded, posterior subtriangular. Valves almost equal. Anterior 

 cardinal angle rounded, posterior cardinal angle obtuse. Greatest width in the 

 posterior third. Subcentral-tubercle distinct. Eye-tubercle rounded and distinct. 

 Ornamentation consists of reticulations dominated by four longitudinal ridges. 

 The dorsal ridge commences just above the subcentral-tubercle and is slightly 

 arched upward (in lateral view over-reaching dorsal margin), the median or second 

 ridge runs almost diagonally from anteroventral margin towards posterodorsal 

 corner, then bending down towards posteroventral margin ; the ventral or fourth 

 ridge (better seen in ventral view) lies between the third ridge and the ventral 

 margin and is not as well-developed as the other three. Anterior and posterior 

 marginal rims high. Anterior margin ornamented with 15-18 small spines, postero- 

 ventral margin with 5-6 relatively large spines. Internal characters unknown. 



Dimensions (mm). 



L 

 Io. 4325 Carapace male (holotype) 0-83 



Io. 4273 Carapace female 0-77 



Io. 3116 Carapace female 077 



Comparison. This species differs from Costa {Paracosta) disintegrata sp. nov. in 

 having well-developed longitudinal ridges and a different outline. Costa {Paracosta) 



H 



w 



o-39 



o-37 



o-39 



o-37 



o-39 



o-37 



