136 PALEONTOLOGY 



complexity or anagenesis of ornament with advancing age, 

 in contrast to the catagenesis of ornament in Asteroceras. 

 It also shows decided change in whorl-shape. 



The body-chamber of these small specimens occupies 

 just over half a whorl, and the last two or three septa 

 are usually crowded together a feature common in the 

 last septa of large ammonites. This suggests that the 

 small size of these ammonites was not due to premature 

 death, but that they were a small species. Larger 

 species of the same genus are for the most part rather 

 later in time, so that the genus shows anagenesis in 

 size as well as in ornament. 



The suture-line of Xipheroceras is (at 20 mm. or less) 

 like a much-simplified Asteroceras suture, except that there 

 are no auxiliary lobes and saddles, and the lateral lobes 

 do not lie so much in front of the peripheral. In older 

 shells the subdivision of the lobes and saddles becomes 

 much more complex. 



These four examples of cephalopods have been chosen 

 for description as easily obtainable, so that readers can 

 verify the descriptions on actual specimens. The next 

 few examples are less common, and will be described 

 more briefly. 



5. Tetrameroceras obovatum (Fig. 40) is one of 

 several more or less ellipsoidal species found in the Lower 

 Ludlow shales of Herefordshire and Shropshire. It is 

 about no mm. long, 58 mm. wide, and 75 mm. high. 

 Preserved as a cast, it is seen that about one-half of the 

 shell is body-chamber, the rest consisting of about nine 

 gas-chambers. The whole shows a very slight curvature, 



