278 



DE. A. E. TRUE MAX OX THE 



[vol. lxxiv, 



AXDROGYXOCERA.S DITARICOSTA, Sp. 110V. (PI. XXII, figs. 1 fl- 



. s.j v l c & text-fig. 8.) 



Dimensions of holotype : — 



Diameter. Whorl height. Whorl thickness. Umbilicus. 



142 mm. 42 per cent. 57 per cent. 27 per cent. 



Q, Whorls swollen, nearly round, sides slightly flattened, moderately 

 involute ; flattened costse with two rows of tubercles. From the 

 umbilical margin the ribs slope back to the inner tubercles ; between 

 the tubercles thev often divide, usuallv uniting as^ain at the outer 

 tubercles, splitting into two or three ribs which pass directly across 



Fig. 8. — Sutural development of Androgynoceras divaricosta 



(Lincoln) . 



100 mm. 



8 mm. \/ 



the periphery. Tubercles usually paired, but the outer are slightly 

 more numerous than the inner (imparinode stage commencing, 

 34/29). 



The tubercles of the type-specimen are rounded knobs, but in 

 specimens with a well-preserved shell they are moderately sharp. 

 Longitudinal striatums are present. Involution does not reach the 

 inner row of tubercles. 



Development. — The form of the shell throughout life changes 

 veiy little, the whorls being nearly round and merely increasing in 

 size. The shell is quite smooth up to a diameter of 4 mm., after 

 which it has closely- set ribs for about a quarter of a whorl ; 

 bituberculation commences at a diameter of 20 mm. 



