14 



GEOLOGICAL MEilOlRS. 



latter section, it is subdivided into four categories, two pro\isional 

 (transitoires) groups being adopted for all those forms which, having 

 lost their shell, have lost, at the same time, the means of distinction. 

 These categories are very unequal, some of them containing a very 

 considerable number of Orthoceratites. But in comparing the species 

 according to the most marked af&nities of the ornaments of the sur- 

 face, it is easy to subdivide each category into different groups, ana- 

 logous to those which L. von Buch has established among the Am- 

 monites, under the name of families. 



It has been thought convenient by the author to leave out of these 

 categories, which are principally founded upon the Orthoceratites of 

 the Third Silurian fauna, the subgenus Endoceras, Hall, which con- 

 tains species exclusively belonging to the Second fauna, and which is 

 strongly characterized by the form of its siphon. 



In the same way, the peculiar conformation of the siphon in the 

 fossils originally named Huronia by C. Stokes confirms the author 

 in maintaining them as a subgenus under the same denomination. 

 The horizon from which these fossils come, in the Drummond Isle of 

 Lake Huron, is stated to correspond with that of the Clinton and 

 Magara group in the State of Xew York — that is to say, to the first 

 phase of the Third fauna. But this stratigraphical assimilation wants 

 confirmation. The forms named Gonioceras, Hall, are also grouped 

 as a subgenus, on account of their very flattened transverse section 

 and the undulations of the suture of their chambers. 



With regard to those Orthoceratites possessing a siphon composed 

 of spheroidal elements of variable appearance, of which the principal 

 types are 0. cochleatum, Schlot., and 0. nummularium, Sow., M. 

 Barrande states that he has unsuccessfully attempted to assign to this 

 class its proper place in that subdivision of Orthoceratites which pos- 

 sesses a siphon more or less straight and cylindrical. This difficulty 

 is produced by the gradual transition between the nummuloid and 

 the cylindiical formation of the elements of the siphon among the 

 Orthoceratites ; and it necessitates the distribution, of those of the 

 Orthoceratites of Bohemia which are connected with the type of 

 forms called CocJiIeati, among the different groups, according to the 

 appearance of their ornamentation. 



Classification of the Orthoceratites of Bohemia. 





Sections. 



Categories. 



t 



1 





( 



I. Short-coned 



Orthoce- I 

 ratites. J 



II. Long-coned 



Orthocera- - 

 tites. 



Transverse or- 

 naments. 



Frovisio7ial 

 groups. 



Shell unknown. 



Longitudinal 

 ornaments 

 predomina- 

 ting. 



1 



2 



3 

 4 



Horizontal or transverse stri®, more or less 

 regular. 



Contour of transverse section triangidar. 

 0. triangulare. 



Transverse section circular or elliptical. 



Longitudinal ornamentation as ridges, fur- 

 rows, edges, bands, strise, or lines. 0. 

 Doricum, Barr., 0. Bacchus, Barr., 0. 

 spectandum, Barr., 0. origincde, Barr., 

 0. Woodwardi, Barr. 



