MUSEUM OF COMPARATIVE ZOOLOGY. 69 



other, with a flatter abdomen, it lies against one side of the first whorl, 

 occupying but a very sm^ill portion of the entire space.* Here there 

 can be but little doubt of the specific value of the differences which are 

 manifested, not only in the youngest stage, but throughout life, and 

 affect not the symmetry of the orbit of revolution, but the form of the 

 whorl itself. 



In all of the seven species observed by Sandberger, as may be seen 

 by his figures f and in the four distinct species observed by me, count- 

 ing Gon. crenistria as two good species, the differences of the ovisac! 

 are very distinct, whereas in several individuals of the same species, 

 Goniatites crenistria, the so-called variety with a flat abdomen, which I 

 have observed, and in several of Go. atratus, no such differences were 

 observed between the different individuals. The slight differences in 

 the amount of coiling between the young of varieties latidorsalis and 

 ealeuliformis of Go. lamed, figured by Sandberger,} were probably 

 therefore unusual among the Devonian and Carboniferous species. 



The succession also indicated by the foregoing facts is precisely 

 what might have been anticipated from the general morphology of the 

 Tetrabranchiate Cephalopods. 



The range of form has been among the Nautiloids from the straight 

 Orthoceratite through intermediate arcuate genera, to the partially 

 coiled Lituites, and finally the closely coiled Nautilus. Such being the 

 case, if there is any truth in the doctrine of evolution, we must expect 

 to find some reference to the peculiarities of the parent Nautiloid stock 

 in the earlier stages of development among the Ammonoids. And 

 further, as a direct and unavoidable corollary of the above, we ought to 

 find this reference more distinct in the young of the earlier species of 

 Ammonoids, the Goniatites of the Silurian, and less noticeable in the 

 Goniatites of the Devonian and Carboniferous, and, finally, almost 

 obliterated, or, at any rate, still less distinct in the typical Ammonite 

 of the Jura. 



So far as the facts have been ascertained, they all point in this direc- 

 tion. The simple Nautiloid-l'.ke. Goniatites of the Silurian may ex- 

 hibit an Orthoceratiticor straight form, or be closely coiled in the young 

 of different varieties of two distinct species. A species therefore, on 

 this horizon, may have a range of variation in form, during the earlier 



* Plate III, Fig. 7. f Piute I, Figs. 11, 18. J Plate I, Figs. 15, 1G. 



