62 BULLETIN OF THE 



species, and prepares the observer to find a common fixed type of form 

 in the young of the Jurassic species of Ammonites, •which belong to 

 the same order and are intimately connected by transitional genera. 



Everywhere the closest correlation is traceable between the amount 

 of coiling, the amount of involution or envelopment of the whorls, 

 the form of the whorl, and the development of the septa, — the simpler 

 sutures being invariably associated with shallow concave slowly chang- 

 ing septa, an elliptical form, and open coils. The more complicated 

 sutures, whose characteristics change more cpjickly in course of growth, 

 are combined always with a more or less crescent-shaped whorl, and 

 closely embracing or involving coils, whether found among Nautiloids 

 or Ammonoids. 



This agrees with the correlations of the structure and morphology 

 of the adults as worked out by Bronn, Barrande, and others, among 

 Nautiloids and Ammonoids, from the straight Orthoceras to the coiled 

 Nautilus, and inversely, among Ammonoids, from the closely coiled 

 Goniatites and Ammonites to the straight Baculites ; the general 

 morphology being readily and accurately expressed as a coiling up 

 of a straight cone and the subsequent uncoiling of the same at later 

 stages of the earth's history. The shells are almost universally classi- 

 fied in accordance w T ith this coiling and uncoiling, with which also the 

 structure of the siphon and septa are more or less correlative. 



The siphon in Nautilus, Goniatites and Ammonites terminates, or 

 rather begins, with a blind sac, or siphonal coecum. 



In Nautilus, this occupies a central position somewhat nearer the 

 ventral than the dorsal side, and is enclosed by the shell at the apex 

 of the first whorl. In Goniatites and Ammonites the siphonal coecum 

 has a cone-like prolongation, which appears to open into the bottom 

 of the siphonal coecum in Goniatites. The siphon is developed ear- 

 lier in the last two than in Nautilus, since it is found within the neck 

 of the ovisac. 



The ovishell consists, in Ammonites and probably in Goniatites, of an 

 inner lining layer similar to that of tic chambers of the whorls, and an 

 outer thicker layer. They both extend entirely around the ovisac, but 

 the outer layer is very much thicker on the sides and abdomen. The 

 inner layer bends inward and forms part of the under side of the first 

 septum. The outer layer overlaps the inner and outer layers of the 

 abdomen and sides of the first whorl, showing that here was the mouth 



