MUSEUM OF COMPARATIVE ZOOLOGY. 77 



considerably narrower.* Before the completion of the first revolution, 

 estimating from the neck of the ovisac until the whorl again touches 

 this point of origin, it reassumes the normal rate of increase. 



The increase in the dorso-abdominal diameter appears to be very- 

 marked at first, and this gives a peculiarly broad aspect to the sides 

 of the whorl, just beyond the embryonal umbilicus ; f subsequently the 

 increase is constant and invariable in all the diameters. 



The form of the whorl is notably distinct from what it auerward 

 becomes, and is identical with that of a typical adult Goniatite, and an 

 equally close representative of the young of G. diadema and other 

 closely coiled Goniatites. There is the same broad, even, somewhat 

 flattened curve to the abdomen, and abrupt sides. The retention of 

 this Goniatitic outline is greater or less in different species, and it is 

 necessary to be cautious in estimating the duration of this period of 

 growth. The septa give an accurate measure of the time during which 

 the young animal may be said with truthfulness to have resembled an 

 adult Goniatite in some of its characteristics. 



The types I have examined are evidently too far removed from the 

 Goniatites in the structure of the adults, and their development con- 

 sequently too much accelerated, for any very extended or exact ref- 

 erence, in all their characteristics, at any one period of growth. Such 

 precise identity of the young with the adults of the parent type can 

 only be expected in the Clydonites, or some of the simpler transitional 

 groups, where the Goniatitic stage must necessarily be of longer con- 

 tinuance. 



The outline and septal structure of the young are almost identical 

 between the Goniatites and Nautiloids, more especially the arcuate 

 forms of the young with plain concave septa, such as G. compressus, 

 when compared with the arcuate forms such as Cyrtoceras. Sand- 

 bergei*'s figures show in section the young whorl of this species, which 

 is a regular ellipse, and has an outline and septa very like Nautilus 

 Bohemicus ; the siphon also being at an earlier period undoubtedly 

 abdomino-central, we have here at one stage a close approximation in 

 the general characteristics of the structure. 



The umbilical perforation is very contracted and small in Nautilui 

 Pompilius, when compared with the fossil, and differs also in form from 



* Plate I, Fig. 6. Plate II, Figs. 2, 11. t Plate I, Fig. 7. 



