24 GEOLOGICAL MEMOIRS. 



be arranged by the general form of the shell into two chief groups ; 

 viz. 1. the elongate tapering forms, with an apex of only 2° to 15° Z 

 (fig. 5 a, b) ; 2. the proportionately short forms resembling the 

 phragmocone of the Belemnite in shape, the apex of which forms 

 an angle of at least 16°, and even of 70° (fig. 6 a, b), which latter 

 proportion gives but little length to the shell. In fact, the Bohe- 

 mian species which has such an apex is at the most only 6 to 7 centim. 

 in length. 



In the elongate species the siphuncle is very seldom marginal ; it 

 is sometimes central, and sometimes intermediate. In the short 

 species, on the contrary, it is nearly always marginal, as in the 

 phragmocone of the Belemnite ; the central position being quite an 

 exception. 



I believe I have said on another occasion * that we do not find in 

 Bohemia, either amongst the Orthocerata of ray " Third Fauna," 

 nor the few species of my " Second Fauna," the large and generally 

 excentric siphuncle which characterizes many species of the " Second 

 Fauna " in Sweden, Russia, and North America. The Orthocerata, 

 therefore, as well as the Trilobites, can furnish general characteristics 

 for the distinction of the different Silurian Faunas. 



6. Cyrtoceras. — I have already indicated that the species of this 

 genns can be divided according to the position of the siphuncle 



Fig. 7. 



Cyrtoceras. 

 a, siphuncle dorsal: b. Siphuncle ventral. c. Siphuncle central. 



(fig. 7) — dorsal, ventral, or central — into three groups, which have, 

 however, so little difference in external aspect, that one must see the 

 siphuncle itself to be able to determine the place of a species. The 

 shells of the Cyrtocerata of the Third Fauna have in general but 

 little ornamentation, although in a few cases it is sufiicient to cha- 

 racterize the species. The other elements for specific distinction are 

 of the same nature as those which I have already mentioned in the 

 case of Orthoceras. The species belonging to the group with central 

 siphuncle have generally a circular cross-section, wherein they ap- 



* [See Quart. Journ. Geol. Soc. vol. viii. Miscell. p. 33.~Ed.] 



