1871.] 397 [Hyatt 



In the Devonian among the young of Goniatites the variation of 

 the young is not so great, and according to Sandberger's observation 

 the amount of coiling is at least of specific value. 



In the Jura the young do not vary among the typical Ammonites, 

 all are closely coiled and all are involute. 



The straight young of the Silurian Goniatites Bohemicus are evi- 

 dently reversions to the ancestral type of Endoceras, and show the 

 tendency of the organization to revert to that type whenever the 

 growth is retarded ; whereas the closely coiled varieties of the same 

 species show the progressive tendency of the series, which is expressed 

 more determinately among the Goniatites of the Devonian, and finally 

 becomes universal among the typical Ammonities of the Jura. The 

 first septa of Ammonites and Goniatites closely resemble each other; 

 and also those of the adult Nautiloids; both have large, simple, entire 

 abdominal cells, and broad, simple, superior lateral lobes. The form 

 of the first whorl among the Ammonites is like that of a typical adult 

 Goniatite, but the septa cannot be so closely compared. 



The siphon is very similar in both forms. It is a blind sac, at first 

 opening at the middle through the first septum. The second septum 

 embraces the narrowing neck, from which springs the slender tube of 

 the siphon proper. Every successive part between the different septa 

 is formed by a posterior inflection or bending of the septa themselves. 

 Thus the fundus of the siphon is composed of a coecal prolongation 

 of the first septum ; this is the lower part of the siphonal coecum or 

 bag. The next segment is formed by a corresponding elongation of 

 the second septum, which, however, is open at the bottom, and con- 

 tinues the walls of the blind sac ; the next, a third septum, sends down 

 another prolongation, which is very narrow comparatively, and forms 

 the neck of the siphonal coecum or the siphon proper. The differ- 

 ences of Ammonites and Goniatites are to be found in the conical 

 prolongation of the siphonal coecum. This organ is a cone-like pos- 

 terior extension of the coecum, which opens into the bottom of the 

 siphonal coecum in Goniatites, but is closed over by the bottom of the 

 coecum- in Ammonites. 



The second septum of Goniatites is like those of the adult Nau- 

 tilini of the Silurian ; the third has a pointed goniatitic superior lateral 

 lobe. The Ammonite has the second septum more ammonitic than 

 goniatitic on account of the broad superior lateral and abdominal 

 lobes. There is here a decided acceleration, since the true goniatitic 

 characteristics are hardly visible except in the form, whereas in Gon- 



