BARRANDE SILURIAN PTEROPODS. 



19 



Hemiceras by these internal concentric layers did not prevent 

 Bronn, in 1849, from placing them in their present position ; and 

 in 1863, Mr. E. Billings and Prof. J. D. Dana followed his example 

 ^vith the fossils named Salterella, which present an analogous struc- 

 ture of their shells. M. Barrande considers that these views are 

 well borne out by the observations which he published in 1855, 

 upon the existence of an organic deposit, in the form of concen- 

 tric plates, obstructing the internal cavity of Conularia fecunda. 



The existence of partitions in certain species of the different 

 genera of Pteropods, establishes a connexion between their shells 

 and those of the Cephalopods. But the constant absence of the 

 siphon in the fii-st-named order prevents its being confounded with 

 the second, of which all the shells are invariably provided with this 

 organ. 



The same preponderance which has been observed in the number 

 of genera of Pteropods which Bohemia possesses applies in like 

 manner to the number of species ; for while in the IJnited States, 

 the British Isles, and Russia, the greatest SHurian regions of the 

 northern zone, 69 species may be enumerated, Bohemia possesses 

 no less than 68 distinct species, or 27 per cent, of the total number 

 of known forms (246) in the Palaeozoic world. 



Table of the Distribution of the Genera of Pterojjods in the luhole of 

 the tivo great Palceozoic Zones. 





Silurian 

 Faunas. 



'2 



o 



> 



i 

 1 



o 



II 



i 



S3 



I. 



II. 



III. 



Conularia, Mill 



Hyolithes, Eicliw 



Pterotheca, Salt 



Phragmofheca, Barr 



Coleoprion, Sanclb 



Tentaculites, Schloth 



Styliola, Lesu 





7 



4f 



4;- 

 ■ss- 



-A- 



■Jf- 



z 





- 



j 



■!«• 



Salterella, Bill 



Hemiceras, Eichw 



1 



-1 



The remarkable concentration of the species of tliis order in the 

 Silurian system, and their rapid diminution in the subsequent 

 systems, are in complete harmony with the results obtained from 

 the author's study of the Cephalopoda. On the other hand, while 

 the epoch of the maximum specific development of the Palaeozoic 

 Nautilidas corresponds to the Third Silurian fauna, that of the greatest 

 specific richness of the Pteropods is manifested in the Second Silurian 

 fauna. 



In comparing the two great Palaeozoic zones with regard to the 

 number of species of Pteropods which arc proper to them, it is found 

 that the northern zone has furnished 161, while but 80 arc known 



VOL. XXIII. PART II. V 



