196 



MANUAX OF THE MOLLUSCA, 



an arch over the aperture and dividing it into two outlets, one 

 corresponding with that above the hood of the nautilus, which 

 gives passage to the dorsal fold of the mantle ; the other witt 

 that below the hood, whence issue the tentacles, mouth, and 

 funnel ; such a modification, we may presume, could not take 

 place before the termination of the growth of the individual."* 

 (Owen.) 



M. D'Orbigny has figured several examples of deformed 

 ammonites, in which one side of the shell is scarcely developed, 

 and the keel is consequently lateral. Such specimens probably 

 indicate the partial atrophy of the branchiae on one side. In 

 the British Museum there are deformed specimens of A. ohtusus, 

 amaltheus, and tuhercidatus. 



Fig. 60.t 



GoNiATiTES, De Haan. 



Etymology, gonia angles, (should be written gonialites ?). 

 Synonym, aganides, D'Orbigny (not Montf= ^^wna zic-zac). 

 Examples, Q-. Henslowi (PI. III., Pig. 1), Gr. sphericus (Figs. 

 60 and 46). 



Shell discoidal ; sutures lobed ; siphuncle dorsal. 

 Distribution, 197 species. Upper Silurian — Trias. Europe. 



Ehabdoceras, Hauer, 1860. 



Shell straight, orthoceratoid, with bold sculpture, 

 with rounded lobes. 



Distribution, 1 species. Trias. Germany. 



Septa 



* This unique and abnormal specimen is in the cabinet of S. P. Pratt, Esq. 



t Fig. 60. Goniatites sphericus, Sby. Front and side views of a specimen from the 

 carb. limestone of Derbyshire, in the cabinet of Mr. J. Tennant; the body-chamber 

 and shell-wall have been removed artificially. 



