136 Genera of Fossil Cephalopoda. 



found in Canada. The typical form of tlie shell of animals of this 

 division is a hollow cone, divided into numerous compartments hy 

 transverse partitions called septa, which are penetrated and con- 

 nected together by another hollow cylinder or tube called the 

 siphon or siphuncle. Starting- vvith this idea we have the follow- 

 ing modifications of the shell in the different genera : 



1. Orthoceras. — In this genus the shell is straight, the trans- 

 verse section either circular, oval or more or less triangular, and 

 the siphon either central, marginal, or between the margin and 

 the centre. Fig. 2, represents a short very rapidly tapering 

 form. The species as yet discovered in this country are all long 

 and very gradually tapering. 



2. Cyrtocerai,. — This genus is the same as the former, but is 

 curved, and the siphuncle is in the greater number of species 

 situated near the mai'gin on the side of the convex curvature as 

 shewn by the dotted line in fig. 3. It is sometimes, however, 

 central or even near the other or concave side. 



3. Nautilus. — Same as Cyrtoceras, but so closely coiled that 

 the whorls are all in contact, Fig. 7. In these three genera it 

 will be observed that the form of the aperture is the same as the 

 tranverse section of the tube, but in the next four it is not. 



4. Gomphoceras. — Straight, the same as Orthoceras, but with 

 the aperture contracted in the middle so as to consist of two 

 lobes. The position of the siphon is equally vaiiable with Ortho- 

 ceras. Fig. 1. 



5. Phragmoceras. — Fig. 4. Same as Gomphoceras, but with 

 the shell curved. The siphon is usually on the inside, but species 

 have been found with it difierently situated. 



6. Lituites. — This genus has the contracted aperture of the 

 last two ; a portion of the shell is closelj^ coiled and the remainder 

 free and straight. Fig. 6. 



7. • Gyrocera?,. — Fig. 5, represents one form of this remarkable 

 genus. The shell has no straight portion, and in some of the 

 species it is closely coiled as in Nautilus. The siphuncle in the 

 species as yet described is between the centie and the outside. 

 The aperture has been detected in some species found in Bohemia, 

 and is thus described by M. Barrande. In noticing G. mirumy 

 a species whose surface is decorated with spines ari'angedin rows, 

 he says : "It is not, however, on account of the ornament that I 

 have named this species as above ; but the mouth of the shell 

 appears to me very wonderful, being neither round nor elliptical, 



