50 GENERIC TVrES. 



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

 coiled cephalopods start witli a straight or bent coue, and begin 

 their coil abrupth', always leaving an opening in the umbilicus 

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

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

 the perfectly straight Orthoceras to the Nautilus Fompilvus, 

 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 wdth the straight 

 J5aculites 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 cephaloj^oda ; and in a richly illustrated 

 memoir, published by the Museum of Comparative Zoolog}', at 

 Tambridge, 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 eephalop(jds, differs in toto 

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

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

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

 ing eonclusions : 



I. Generic 'I'l/jies. 



J. Absence of cephalopods in the primonliul siliirian 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 heteropoda in 

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



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

 as])ect of the second Silurian fauna. 



This sudden al)pearance is as inexplicable as their total 

 absence in the [)rimordial fauna. This number, 12, consti- 

 tutes nearly half of the 2(5 types admitted in liis studies, 

 among the 3 families: Nautilidje, Ascoceratidte and Gonia- 

 tidje. 



* /'roc. Bost. Soc. N. II., xii, 216, 18G8, 



