28 



Guide to the Invertelrata, 



Trocho- 

 ceras. 



GALLERY 



VII. 

 Table-case 

 71. 



Wall-case 

 7, Table- 

 case 71. 



In Trochoceras the shell is not coiled in one plane, but is 

 helicoidal. It ranges from the Cambrian to the Devonian, but 

 attains its greatest development in the Silurian formation, the rocks 

 of this age in Bohemia, England, and the United States containing 

 abundant species. The Ordovician genus Trocholites consists of 

 three or four involute vrhorls, and is coiled in one plane ; it 

 has been found in North America, Europe, and possibly also in 

 India. 



Gyroceras is coiled in one plane, but the whorls only just touch 

 each other, or are completely out of contact. It ranges from the 

 Silurian to the Carboniferous. Some of the Devonian forms from 

 Germany and South Devon are ornamented with regularly- 

 arranged, strong, tubercular folds ; while some of the Carboniferous 

 forms possessed numerous longitudinal ridges, studded with small 

 tubercles. 



The discoidal, somewhat evolute shell Hercoceras (Eig. 50 \ is 

 at present known only from the Devonian rocks of Bohemia ; the 





Fig. 50. — IT^roc^ras mw-wm, Barrande. (After Barrande.) Devonian: Bohemia. 



Table-case 

 71. 



aperture of the shell is partially closed by the infolding of its inner 

 portion ; its whorls are ornamented with a row of spines or strong 

 tubercles arranged longitudinally. 



Barrandeoceras is a discoidal, involute shell ranging from the 

 Ordovician to the Silurian ; in England it is found in the Silurian 

 rocks of Shropshire and Worcestershire. 



Biscites, with its compressed, evolute whorls (Eig. 51), is limited 

 to the Carboniferous rocks ; numerous examples of the genus have 



