172 PAL/EONTOLOGY 



family in which involute, compressed forms with acute 

 periphery predominate is the Pinacocevatidce, in which the 

 suture ranges from ceratitic to the most highly complex 

 kind known in any arnmonoid. These are the principal 

 Triassic families, none of which survive that period. 



The classification of the Jurassic ammonites is in a 

 very unsettled state at present. Mr. Buckman, the chief 

 English investigator of them, proposes the following 

 provisional scheme (the terms involved are explained 

 farther on) : 



I. Suture-line phylloidal, no keel, no aptychus or anap- 



tychus. Families Phylloceratidtf, Lytocevatidce. 



II. Suture-line phylloidal, passing to complex and much 



inflected ; no keel in most, when present it arises 

 late, after elaboration of suture-line ; with aptychus 

 or anaptychus. 



III. Suture-line with saddle of ceratitic outline, more or 

 less serrated but generally remaining fairly simple ; 

 keel usually developed at a very early stage ; elabora- 

 tion of suture-line, if any, coming later ; with aptychus 

 or anaptychus. 



I. While most of the Mesozoic families of ammonites 

 are short-lived, rapidly attaining their acme and dying 

 away in a single geological age or little more, the two 

 families of this branch are exceptional in enduring with 

 little modification through the Jurassic and Cretaceous 

 periods. They are also peculiar in having a narrower 

 geographical distribution than most other families, 

 their headquarters being in the subtropical waters of 

 " Tethys " the ocean which occupied the site of the 

 present Alpine- Himalayan mountain-systems and the 

 Mediterranean region. Only at intervals of time did a 

 few representatives succeed in migrating into the colder 

 seas of what is now Central and Northern Europe. The 



