284 



DE. A. E. TEUEMAN ON THE 



[vol. lxxiv,. 



Suture simple ; first lateral lobe narrow, external saddle wide 

 with three unequal folioles (fig. 11 i t below). 



Development (PL XXV, figs. 2a-2h). — Specimens studied 

 were from the Oistoceras sub-zone, Waddington, near Lincoln 

 (7, p. 104). 



The protoconch is about 0*63 mm. long, and is of normal shape 

 with a narrow azygous saddle. Ribs appear at a diameter of 



Fig. 11. — Sutured development of Oistoceras omissum {Lincoln). 

 Specimen figured in JPl. XXV, Jig. 2. 



16 mm. 



h 

 10 mm. 



6 mm. 



2-8 mm. 



e 

 2 mm. 



d 

 1*5 mm. 



c 

 4th. 



b 

 3rd. 



a 

 1st. 



2 mm. at the commencement of the second whorl, but do not 

 cross the venter until a diameter of 9 mm. is reached. The ribs 

 have an arrow-like forward projection on the periphery. Through- 

 out development the ribs are fine and regularly spaced, the number 

 of ribs to a whorl increasing steadily with growth ; thus, at a 

 diameter of 30 mm., there are usually thirty ribs on the last whorl. 

 In some species {Oistoceras Jigulinum) the number becomes 

 reduced in the adult or in old ao-e. 



