42 EARLY TERTIARY OSTRACODA 



Description. Sexual dimorphism moderate ; the males are proportionally longer 

 than the females. Carapace subrectangular in lateral outline. Anterior margin 

 broadly rounded, posterior margin almost rounded in left valve but with a slight 

 concavity in the posterodorsal slope of the right valve. Dorsal margin almost 

 straight but appears slightly convex in lateral view due to the dorsal ridge, which 

 slightly over-reaches it ; ventral margin slightly concave in front of the middle. 

 Valves almost equal. Subcentral-tubercle and eye-tubercle present but not pro- 

 nounced. Surface reticulate ; the reticulae concentrically arranged around the 

 subcentral-tubercle. There are three longitudinal ridges, a dorsal ridge curved 

 convexly upward, a median or second less well-developed ridge and a ventral or 

 third ridge which runs obliquely from above the anteroventral corner towards the 

 posterior and is curved convexly downward in its anterior portion. Anterior and 

 posterior marginal rims distinct. Anterior and posterior margins denticulate. 

 Internal features not seen. 



Dimensions (mm). 



L H W 



I°- 433 2 Carapace male (holotype) 0-85 0-46 0-44 



Io. 4282 Carapace female 0-83 0-46 0-46 



Comparison. This species resembles Gyrocythere parvicarinata sp. nov. but differs 

 from it in being larger. Moreover, the posterior in G. parvicarinata is subacuminate. 

 G. grandilaevis is perhaps ancestral to Gyrocythere perfecta sp. nov. which is smaller 

 and has stronger ornamentation. 



Gyrocythere mitigata sp. nov. 



(Plate 21, figs. 5-1 1 ) 



Derivation of name. Latin mitigatus, mellowed ; with reference to the orna- 

 mentation, less emphatic than in the typical species of G. exaggerata. 



Diagnosis. A large, strongly reticulate species of the genus Gyrocythere with 

 three longitudinal ridges, the median ridge almost ill-defined. 



Holotype. Io. 4333, a male carapace (P. 21, figs. 5-8). 

 Paratypes. Io. 4283 + Io. 3119. 



Material. 9 specimens from the locality below from two horizons (sample nos. 

 24131 and 24132). Io. 4283. GSPBM 2547-8. 



Type locality. Zao River section. 



Type horizon. Lower Chocolate Clays, sample no. 24131. 



Description. Sexually dimorphic ; the males are longer than the females. 

 Carapace subrectangular in lateral view. Anterior margin broadly and evenly 

 rounded, dorsal margin almost straight in reality but appears slightly convex in 

 lateral view because of the over-reaching of the dorsal ridge ; posterodorsal slope 

 very slightly concave particularly in the right valve, posterior extremity rounded, 

 postero ventral margin curved, ventral margin almost straight. Greatest height in 

 the anterior quarter, greatest length almost in the middle. Anterior and posterior 



