196 MANUAL 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 with 
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 indiyidual.’’* 
(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 branchize on one side. In 
the British Museum there are deformed specimens of A. obtusus, 
amaltheus, and tuberculatus. 
Fig. 60.+ 
GONIATITES, De Haan. 
Etymology, gonia angles, (should be written gonialites ?). 
Synonym, aganides, D’Orbigny (not Montf=Aturia zic-zac). 
Examples, G. Henslowi (Pl. IlI., Fig. 1), G. sphericus (Figs 
60 and 46). 
Shell discoidal ; sutures lobed ; siphuncle dorsal. 
Distribution, 197 species. Upper Silurian—Trias. - Europe. 
RHABDOCERAS, Hauer, 1860. 
Shell straight, orthoceratoid, with bold sculpture. Sepia 
with rounded lobes. 
Distribution, 1 species. Trias. Germany. 
* This unique and abnormal specimen is in the cabinet of S. P. Pratt, Esq. 
} 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. 
