THE CEPHALOPODA 135 



Whorl - section reniform (kidney- shaped) in inner 

 whorls, becoming higher and less broad later. Amount 

 of indentation one-tenth, of inclusion one-sixth (at 20 mm. 

 diameter). When the umbilicus is clean there can be 

 seen in the centre a small globular pwtoconch, with six 

 whorls around it (up to 20 mm. diameter). The first 

 two whorls or so are quite smooth, then fine striae 

 appear; after another whorl and a half obscure ribs are 

 seen, and about the end of the fourth whorl well defined 

 ribs appear. At the 20 mm. diameter the ribs are about 

 2 mm. apart, and run as follows : starting in the steep 

 inner area they first slope backwards, then curve round 

 to a truly radial direction on the umbilical margin, from 

 which they curve very gently to a slight backward 

 slope on the outer half of the lateral area ; at the peri- 

 pheral margin they swing forward and cross the periphery 

 without interruption in a forwardly-convex curve, but at 

 the same time they become much broader and lower (as 

 though modelled in a plastic substance and then pressed 

 down). The fine striae on and between the ribs follow 

 the same direction, which shows that here as in Asteroceras 

 there was no hyponomic sinus to the aperture but a 

 rostrum, though not a very prominent one. Larger 

 specimens of this (or nearly allied) species show that 

 soon after attaining 20 mm. diameter the ribs begin 

 to show signs of tubercles at the peripheral margin, and 

 in some the flattened peripheral rib tends to divide into a 

 bundle of three ribs. In another species (X. ziphus) the 

 tubercles are developed into long spines. Thus the early 

 species of Xipheroceras show an increasing strength and 



