218 DISTRIBUTION OF THE MOLLUSCA IN TIME. 



characteristic of beds, is that their occurrence is never repeated. 

 They become extinct or modified. " One of the proofs that 

 species are extinct consists in the fact that they have been 

 replaced by other species having filled the same functions and 

 found in deposits formed under similar conditions " (Forbes). 



" These ideas have been combatted by the advocates of the 

 doctrine of colonies, who admit the reappearance of species after 

 the lapse of a period more or less long, and at different levels. 

 But folds, dislocations and faults may simulate the reappearance 

 of an identical fauna. In this case, colonies would be, as 

 d'Archiac has written, only stratigraphical illusions, resulting 

 from an incomplete appreciation of facts, judged by deceptive 

 appearances. ' From the moment when a type has appeared for 

 the first time, to the moment when it has entirely disappeared, 

 there has never been an interruption in its existence' " (Pictet). 



The very ancient and nearly "extinct order of tetrabranchiate 

 cephalopods has been the subject of mitch discussion between 

 the creative and development schools of naturalists, and may 

 be conveniently adduced for the purpose of exhibiting their 

 respective lines of argument. This work would be enlarged to 

 undue proportions were I to attempt to discuss the subject at 

 length : a few pages extracted from Vol. I of my " Manual of 

 Conchology " will give an idea of it : 



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

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

 their coil abruptl}'^, always leaving an opening in the umbilicus 

 throuoh 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 Zoolog}^ at 

 Cambridge, Mass., he attempts to prove the development theorj^ 

 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 IStt) " Etudes 

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

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

 lowing conclusions : 



* Proc. Bost. Soc. N. H., xii, 31 fi, 1868. 



