THE CEPHALOPODA 155 



margin. The periphery is arched and cannot be marked 

 off from the lateral area. The greater part of the 

 surface is marked by very fine striae, which on the lateral 

 area are straight and radial but cross the periphery 

 in a forwardly-concave curve, indicating a much 

 shallower hyponomic sinus than in the Devonian forms 

 already described. The striae are not uniform, but are 

 arranged in bundles, which in passing from the periphery 

 towards the umbilicus take on the character of ribs and 



FIG. 44. GASTRIOCERAS CARBONARIUM (VON BUCH), WESTPHALIAN 



(COAL MEASURES), LANCASHIRE. 



(Natural size.) 



Immature ; two constrictions shown ; the striate ornament indicates a 

 hyponomic sinus (right-hand figure). (Original.) 



at the umbilical margin project slightly as blunt tubercles. 

 At intervals of about half a whorl there occur " constric- 

 tions " i.e., depressions of the shell, i mm. or more in 

 width, following exactly the course of the ornament- 

 lines. 



These constrictions are analogous to the varices of 

 gastropod shells : each one was originally formed just 

 behind the apertural margin (as in Orthoceras decipiens, 

 though in that species there is only one), and it may be 

 presumed that its formation was followed by a period of 



