THE CEPHALOPODA 165 



The shape of the whorl-section varies greatly : it may 

 be rounded, quadrate, cordate, sagittate, depressed, etc. (Fig. 49). 

 These various forms, in combination with different degrees 

 of involution, of rapidity of growth, and of change with 

 advancing age, give rise to an enormous variety of shell- 

 forms, to some of the commonest of which special names 

 are given. Thus, when increase of size is slow, so that 

 for a given diameter there are many whorls, and there is 

 a wide, shallow umbilicus and a square periphery, the 

 resemblance to a cart-wheel suggests the term rotiforni ; 

 but with a rounded periphery, as though the wheel were 

 pneumatically-tyred, the term planulate is used (Fig. 49, 

 a, b}. Discoidal means much compressed with an acute 

 periphery, usually with very small umbilicus. When the 

 whorls are depressed, with broad periphery and rather 

 deep umbilicus, the ammonite if placed on its side suggests 

 the shape of a crown, especially if (as is common) it has 

 a row of tubercles or spines along the lateral margin : 

 such an one is coronate (Fig. 49, /). Mr. Buckman uses 

 the term oxycone for discoidal forms and those approaching 

 them (Fig. 49, c, d}, this being a stage which corresponds 

 to the sphaerocone but differs from it in being com- 

 pressed ; and cadicone for the extreme form of the coronate 

 type, with very deep conical umbilicus (Fig. 49, ). 



FIG. 48. SEPTAL SDTURES OF JURASSIC AND CRETACEOUS 

 AMMONOIDEA. 



Arrangement as in Fig. 47. Circles in e denote the position of tubercles. 

 a, Paltopleuroceras, Domerian ; b, Grammoceras toarcense, Yeovilian ; 

 c, Oppelia, Upper Jurassic ; d, Caloceras, Hettangian ; e, Deroceras, 

 Charmouthian ; /, Cosmoceras, Callovian ; g, Hoplites, Lower Cre- 

 taceous; h, Perisphinctes, Upper Jurassic; i, Tissolia, Upper Cre- 

 taceous; j, Metengonoceras, Upper Cretaceous, a-h, after d'Orbigny; 

 i, after H. Douville" ; /, after Hyatt. 



