part 4] 



EVOLUTION OF THE LI PAROCEKATI D.E. 



281 



Development (PI. XXIV, tigs. 2a & 2 &).— Studied in a 



specimen from the dmdalicosta sub-zone, Lincoln. The innermost 

 whorls are not usually preserved : at a diameter of 2 mm. the shell is 

 smooth with a depressed whorl, ribs appearing as folds on the lateral 

 area at a diameter of 3 mm. This condition continues until over 

 a whorl later, when at a diameter of 7 mm. faint folds cross the 

 periphery with a decided forward curve. The whorl height has 

 increased meanwhile, so that the whorl is almost circular in section ; 

 A. crcseens, from this diameter onwards, is much stouter than any 



Fig. 9. — Sutural development of Amblyeoceras crescens 



(Lincoln). 



other Capricorn described. The ribs are regular and closelv placed, 

 until a diameter of 20 mm. (2-5 to a whorl), after which thev are 

 separated by wider spaces (18 to a whorl). The arrangement of 

 the ribs on the inner whorls thus resembles that in A. breuilobatum. 



Diameter. 



"Whorl height. 



Whorl thickness 



mm. 



per cent. 



per cent. 



2 



45 



64 



3-8 



43 



60 



11 



39 



43 



42 



36 



33 



z2 



