MUSEUM OF COMPARATIVE ZOOLOGY. 61 



the form of a scar or cicatrix. The shell of the neck of the ovisac, in 

 Ammonites and Goniatites, has the same form as the shell of the first 

 whorl, whereas in Nautilus it is evident that the shell of the ovisac 

 roofed over the living chamber, somewhat as in Gomphoceras. The 

 opening into the apex or neck of the ovisac, where it joined the first 

 whorl, was, as shown by the cicatrix, a narrow vertical slit bearing a slight 

 resemblance to the opening of the living chamber in Gomphoceras. 



All of the animals of these three groups undergo a true metamor- 

 phosis in passing from the globular ovisac into the first whorl. 



The first septum occurs at the junction of the ovisac and the first 

 whorl, in both Goniatites and Ammonites, and has an entire abdominal 

 cell with two simple lateral lobes as in Nautilus. 



The second septum in the Jurassic species of Ammonites, which 

 alone were examined,* had no positive resemblance to the adult of any 

 species of Goniatites, but the entire, simple sutures and simple ventral' 

 lobes have a general resemblance to that type, though the superior lat- 

 eral lobes are present and undeniably Ammonitic even at this early age. 



The second septum of the closely-coiled Goniatites has a much 

 shallower ventral lobe than in Ammonites, and no superior laterals. 

 The sutures resemble those of the adults of the Silurian Goniatites, 

 but the ventral lobe is shallower and broader. The superior lateral 

 lobes which are first seen in the third septum have the sharp outlines 

 which distinguish the Goniatites. 



This first whorl, or apical portion of the shell, is closely coiled in 

 Ammonites, more or less involute, and resembles the adults of the typi- 

 cal Goniatites in form. 



The form of the first whorl in Goniatites as well as the development 

 of the septa varies excessively in the different species, or even in 

 different varieties of the same species. In some the form and septa 

 must be very similar to those of the straight Nautiloids, such as Ortho- 

 ceras or Endoceras, while in others we have representatives of the 

 arcuate Cyrtoceras, and finally those which are closely coiled, involute, 

 and hardly distinguishable in external appearance from the young of 

 Ammonites. This variability is, so far as we know, greater in the 

 earlier or Silurian, than in the later or Devonian and Carboniferous 



* Species of simpler adult structure and earlier occurrence, among the Ceratites or 

 Clydonites, would probably present an aspect more like some of the adult Goniatites in 

 their young. 



