642 W. D. SMITH 



Mesozoic ammonites. It is about | mm wide and is calcareous, marked 

 with regular rows of small pustules, as in Lytoceras, and shows very 

 plainly the siphonal caecum and siphonal collars pointing forward. 

 What these pustules mean is a matter, more or less, of conjecture; 

 however, it is quite possible that these may be traces of the orna- 

 mentation of some ancestor, either Goniatites strictus, which in the 

 adult stage shows markings not greatly unlike these, or an ancestor 

 of both. At any rate, they represent a character very remote which 

 has been pushed back now into the embryonic stage. These pus- 

 tules cease abruptly at a constriction in the shell which occurs at a 

 diameter of o.66 mm , or about one revolution. This is shown in Num- 

 ber 13, Fig. 1. 



The protoconch is wider, invariably, than the first coil in all species 

 of Scaphites studied during this work, and this is true also in the 

 case of Baculites and Lytoceras, as will be seen from an examination 

 of the plates in Professor Smith's papers. 



The first septum is angustisellate. According to Hyatt, this is 

 called the ananepionic stage, and shows that the animal has in its 

 ontogeny a stage which in the history of the race is that of a nautiloid. 

 (The sutures are shown in Fig. 3.) 



With the second septum, which is marked by the development of 

 a siphonal lobe, the animal becomes an Ammonoid. This stage is 

 called the metanepionic or second larval period. Right here may 

 be mentioned one thing that indicates the highly accelerated nature 

 of some characters, and the unequal acceleration that may be found 

 in these later Ammonites. In the OrtJioceratida and Naulilinidae 

 the siphuncle is central in the former, and just a little distance away 

 from the center in the latter, migrating gradually in the later Nauti- 

 loids and Goniatites toward the margin of the coil, until in the ammon- 

 ites it is marginal. But here, in the very early stages, which are 

 nautilian and goniatitic, we find the siphuncle marginal, and also the 

 collars prosiphonate (these characters could be made out on only 

 the first four septa). In the early cephalopods the collars are retro- 

 siphonate. So we have very great acceleration of some characters 

 compared with others. This in itself makes it quite probable that 

 the sutures are highly modified by acceleration of characters intro- 

 duced late in the history of the race, and likewise lessens the prob- 



