160 PALEONTOLOGY 



project outwards as a horn, or even be curled backwards 

 (Fig. 52). Genera with a prominent rostrum are found 

 onwards to the Cretaceous period, but in certain Lower 

 Jurassic forms a pair of lateral protuberances (lappets) 

 appear, and as these increase the rostrum diminishes 

 until it may disappear altogether (Fig. 46, a-h). Genera 

 with large lappets are confined to the Upper Jurassic. 

 In many cases the lappets tend to close in the aperture, 

 the extreme case being that of Morphoceras (Fig. 46, *'), 

 where the aperture is even more completely subdivided 

 than in the Silurian Tetrameroceras. 



The constrictions already described in Gastrioceras are 

 found in various other ammonoids, sometimes at intervals 

 throughout life (Fig. 41, d), sometimes only in very early 

 stages of growth. As we have seen in gastropods, all 

 specializations of the apertural margin must have inter- 

 fered with the simple growth of the shell, and we must 

 suppose, either that they were not formed until the shell 

 had attained its full size, or that, if formed, they had to be 

 resorbed before growth could continue. Either of these 

 suppositions may be true in particular cases. In a 

 number of genera constrictions do occur at intervals 

 throughout the length, and in one case (Perisphinctes and 

 allied genera) there are sometimes found peculiar 

 markings (parabola) at intervals which may indicate the 

 places where lappets were resorbed. 



In the body-chamber of some ammonites there is 

 frequently found a pair of symmetrical calcareous plates, 

 which together form a heart-shaped body (aptychus). 

 Similar aptychi are found apart from ammonites, though 



