70 BULLETIN OF THE 



stages of development, equivalent to that occurring among the adult 

 forms of Nautiloids from Orthoceras to Lituites. 



The Goniatites of the Devonian, however, even in such simple 

 species as Go. compressus,* exhibit only arcuate whorls, and in the 

 majority of cases are more or less completely coiled. No elliptical 

 varieties have been observed in the adults, and the variations of form 

 in the young of the same species probably do not exceed what has 

 already been observed. | 



Among the typical Ammonites of the Jura, not only is the young of 

 the same species invariably similar, so far as the coiling and form of 

 the ovisac is concerned, but the young of all the closely coiled or 

 normally formed species are also closely coiled and involute or en- 

 veloped. I 



We find in this a perceptible acceleration in the development of the 

 young precisely proportionate to the estrangement, either in time, or in 

 adult organization, of the Ammonoids from their supposed parent stock. 

 There are evidently two tendencies at variance with each other: one 

 strongly reversionary, appearing in the frequency with which the ear- 

 lier Goniatites repeat the parent form in certain isolated instances in 

 the young of varieties, and in the different species of the later Gonia- 

 tites manifesting itself in the arcuate cone of the young of Goniatites 

 compressus and others of the Nautilini, and in the closer, though non- 

 involute coiling of the young of other forms. Evidently this tendency 

 is losing its power to affect and modify the organization, or, in other 

 words, its prepotency. The other tendency, which is expressed in the 

 closer coiling of the whorls, and finally in their increasing involution, is 

 decidedly progressive, increasing in power to the final and ultimate 

 extinction of all reference to the ancestral type, except in the internal 

 organization. Here, as will be shown, the siphon for a limited time 

 remains central in the first whorl, and the first septum has a large 

 entire abdominal cell, and simply concave lateral lobes, as in the Nau- 

 tiloids. 



The form, however, of the first whorl of the Ammonoid is like 

 Goniatites, the shell similar, and the second septum has the invariable 

 abdominal lobe, superior lateral cells, and lobes of the simpler adult 



* Plate I, Fig. 11. 



t As already noticed between Figs. 15 ard 16 of Goniatites lamed. Plate I. 



t Plate I, Figs. 2, 4, 5, 6, 7, 8. Plate II, Figs. 7 - 9, 11. 



