FROM WEST PAKISTAN 57 



Diagnosis. Reticulate Trachyleberididae with a ventral ridge ; with or without 

 spines or papillae ; mostly with pronounced posterodorsal process. 



Type species. Cythere horrescens Bosquet 1852. 



Description. Sexual dimorphism present in most of the species. Carapace 

 subrectangular to subquadrate in lateral outline. Anterior margin broadly rounded, 

 posterodorsal margin very slightly concave, posterodorsal margin either curved or 

 almost straight, dorsal margin almost straight or slightly convex (but appears 

 irregular in lateral outline in many species due to surface ornamentation), ventral 

 margin slightly concave in front of the middle or nearly straight (over-reached by a 

 ventral ridge in lateral view in some species). Valves almost equal in size although 

 the right valve over-reaches the left at the anterior margin. Subcentral-tubercle 

 more or less well-developed. Eye-tubercle distinct. Surface reticulate with or 

 without superimposed papillae or spines. A posterodorsal process is generally 

 present. A ventral ridge more or less prominent is always present ; it is either 

 straight or slightly curved convexly downward in the middle culminating in the 

 posterior fourth usually in a spine or an ala. Anterior and posterior marginal rims 

 always present, more or less distinct. Radial pore canals simple, almost straight, 

 irregularly spaced, sometimes crossing one another, fairly numerous (approximately 

 30 anteriorly in the type species). Line of concrescence and inner margin coincide. 

 Duplicature fairly wide. Selvage more or less pronounced, submarginal in left valve 

 but at some distance in the right valve. Right valve with a deep and well-developed 

 anterior and ventral flange groove. Adductor scars in a vertical column of four 

 elongate scars with two almost rounded frontal scars (see description of P. horrescens). 

 Hinge holamphidont. 



Comparison. Hirsutocy there Howe 195 1 has a wider duplicature and lacks a 

 ventral ridge. Australicy there Benson 1964 is a much larger genus in which fine 

 pittings occur within the reticulae. The two median adductor scars are also divided 

 into two. Moreover, Australicythere has a posterior vertical ridge and a less promin- 

 ent ventral ridge not ending in a spine posteriorly. Bradleya Hornibrook 1952 has 

 both dorsal and ventral ridges, in this respect it differs from Phalcocythere which has 

 only a ventral ridge. 



Remarks. This genus is so far known from the Eocene of the Paris Basin, West 

 Pakistan, Tanzania and an undescribed species from the Aquitaine Basin. 



Phalcocythere horrescens (Bosquet) 

 (Plate 29, fig. 5 ; Plate 30, figs. 1-6 ; Plate 33, figs. 12, 13) 



1852 Cythere horrescens Bosquet, p. 119, pi. 6, fig. 5. 



1852 Cythere thierensiana Bosquet (pars), p. 98. 



1852 Cythere nebulosa Bosquet, p. 105, pi. 5. fig. 8. 



1955 Trachyleberis horrescens (Bosquet), Apostolescu, p. 272, pi. 8, figs. 125-126. 



1957 Hirsutocy there horrescens (Bosquet), Keij, p. 101, pi. 15, fig. 4 ; pi. 17, figs.6-7. 



Diagnosis. Phalcocythere in which posteroventral margin is straight in left valve 

 but curved in the right with five or six large spines, shell surface ornamented by well- 



