50 GENERIC TYPES. 



Mr. Alpheus Hyatt has remarked that the young of all the 

 coiled cephalopods start with a straight or bent cone, and begin 

 their coil abruptly, always leaving an opening in the umbilicus 

 through the centre of the first whorl. The development of the 

 Nautiloids, in time, is also marked by a gradual involution from 

 the perfectly straight Orthoceras to the Nautilus Pompilius, 

 where the expansion of the last whorl conceals the umbilicus. 

 The progress of the Ammonoids, on the other hand, is marked 

 by the gradual uncoiling of the shell, ending with the straight 

 Baculites of the cretaceous ; this feature is, therefore, of great 

 importance in a natural classification of these groups.* 



Mr. Hyatt has also carefully studied the embryology of the 

 shell of the fossil cephalopoda; and in a richly illustrated 

 memoir, published by the Museum of Comparative Zoology, at 

 Cambridge, Mass., he attempts to prove the development theory 

 by the results of these studies. 



M. Joachim Barrande, however, who is the most distinguished 

 of living authorities upon the fossil cephalopods, differs in toto 

 from Mr. Hyatt's decisions. He has published (in 1877) " Etudes 

 Generales," in which he devotes over two hundred octavo pages 

 to a careful review of the entire subject, and reaches the follow- 

 ing conclusions : 



I. Generic Types. 



1. Absence of cephalopods in the primordial Silurian fauna of 

 all the countries where it has been ascertained to exist ; that 

 is to say in about 25 natural basins, largely spread over the 

 two continents. This absence is in harmony with that of the 

 acephala and the rarity of gasteropoda and hi'tvropodn in 

 the same fauna. It is inexplicable by the theories of evolution. 



2. Sudden appearance of 12 types of cephalopods in the first 

 aspect of the second Silurian fauna. 



This sudden appearance is as inexplicable as their total 

 absence in the primordial fauna. This number, 12, consti- 

 tutes nearly half of the 26 types admitted in his studies, 

 among the 3 families: Nautilida 1 . A scoceratidre and Gonia- 

 tidce. 



* Proc. Bost. Soc. N. II. , xii, 216, 1868. 



