﻿BEITISH FOSSIL CEPHALOPODA. 



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deeper parts of which might compare with the numerous normal lines, but the surface 

 between is convex. They are most common in the group Inflati. 



(e.) Epidermids. — This name is given by Barrande to curious impressed lines 

 seen on the casts of Nautiloids. They are usually transverse, but occasionally 

 longitudinal ; when the former, they are often arranged sigmoidally, but each indi- 

 vidual stria does not go far, but its place is taken by others, and they all run into, 

 each other. Since they are hollow on the cast they are in relief on the shell, to 

 which they give a wrinkled appearance. When this is seen, it is called by D. D. 

 Sandberger the " Runzelschicht," or wrinkled layer, and is most common on the 

 ventral surface of the penultimate whorl, being due to the dorsal layer of the suc- 

 ceeding whorl ; but the transverse epidermids are not confined to any part of the 

 shell. Barrande states that he has seen them on the recent Nautilus. They are 

 obviously due to a similar disposition of the surface of the mantle in the locality 

 where they occur. They are most common in the Trochocerata, and are very liable 

 to be confounded with ornaments. I have never seen, however, a longitudinal set 

 overlying a transverse set, as Barrande has. 



CLASSIFICATION OF THE CEPHALOPODA. 



As the Dibranchiata have no known representatives in the Silurian rocks, the 

 present question resolves itself into a classification of the Tetrabranchiata. Certain 

 groups, however, which do occur in Silurian rocks, have been classed by some 

 authors in the former order, and it is necessary, therefore, to discuss their true 

 position. By M. le CheV. d'Eichwald, the whole of his family of Orthoceratidce, 

 that is, the whole of the uncoiled Silurian Cephalopods, have been so treated. For 

 this there appears no better reason than that the chambered shells of an Orthoceras 

 may be compared with the phragmocone of a Belemnite, and the latter is 

 undoubtedly dibranchiate. It may, however, be equally well compared with the 

 shell of the Nautilus, and it bears the same kind of resemblance to it that the 

 Belemnite does to the Spirula. The great importance of the siphuncle in many 

 Orthocerata, the preservation of colour on their shells, and their ornamental 



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