The Quarterly journal of the Geological Society of London . 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 length. In the elongate species the siphuncle is very seldom marginal; itis sometimes central, and sometimes intermediate. In the shortspecies, on the contrary, it is nearly always marginal, as in thephragmocone of the Belemnite ; the central position being quite anexception. I believe I have said on another occasion * that we do not find inBohemia, either amongst the Orthocerata of my Third Fauna
The Quarterly journal of the Geological Society of London . 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 length. In the elongate species the siphuncle is very seldom marginal; itis sometimes central, and sometimes intermediate. In the shortspecies, on the contrary, it is nearly always marginal, as in thephragmocone of the Belemnite ; the central position being quite anexception. I believe I have said on another occasion * that we do not find inBohemia, either amongst the Orthocerata of my Third Fauna,nor the few species of my Second Fauna, the large and generallyexcentric siphuncle which characterizes many species of the SecondFauna in Sweden, Russia, and North America. The Orthocerata,therefore, as well as the Trilobites, can furnish general characteristicsfor the distinction of the different Silurian Faunas. 6. Cyrtoceras.—I have already indicated that the species of thisgenus can be divided according to the position of the siphuncle Fig. 7. a be. ® 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 thesiphuncle itself to be able to determine the place of a species. Theshells of the Cyrtocerata of the Third Fauna have in general butlittle ornamentation, although in a few cases it is sufficient to cha-racterize the species. The other elements for specific distinction areof the same nature as those which I have already mentioned in thecase of Orthoceras. The species belonging to the group with centralsiphuncle have generally a circular cross-section, wherein they ap- * [See Quart. Journ. Geol. Soc. vol. viii. Miscell. p. 33.—Ed.] BARRANDE—SILURIAN CEPHALOPODA OF BOHEMIA. 25
Size: 2236px × 1118px
Photo credit: © The Reading Room / Alamy / Afripics
License: Licensed
Model Released: No
Keywords: ., bookcentury1800, bookdecade1840, booksubjectgeology, bookyear1845
