Go 
keel is continuous, forming tlie periphery of the inner whorls 
on the upper surface, the innermost ones rising slightly above 
the plane of the second to form the low spire ; ventral surface 
generally concave as a whole, except the body whorl which is 
convex, with an ill-defined spiral obtuse keel. Upper surface 
of the whorls crossed by direct, or very slightly sigmoidal, 
transverse, simple, or occasionally bifurcating costse, crenulat- 
ing the spiral keel, and passing on to the back where they 
either become lost or faint, reappearing at the obtuse keel of 
the inferior side and passing similarly across the remaining 
portion of the whorls. Form of mouth and operculum unknown. 
Obs. — In mature individuals the back broadens, the sides of 
the body whorl become more rounded, the cos tee towards the 
aperture broaden out into folds, and the whole shell puts on a 
nautiloid appearance. Furthermore the cos toe on the lower 
surface become obliterated. These changes have been already 
noticed by l)r. Lindstrom in Oriostoma discors , of which he 
remarks that u the spiral ridges are more prominent in small 
specimens, or on the old whorls of the larger ones, and are apt 
to disappear on the larger and younger whorls. ” The costoe 
bifurcate both at the suture and the bounding keel, and are as 
a rule about their own distance apart, but in passing over the 
keel of the body-whorl they very distinctly impart a crenulated 
appearance. 
Although resembling Oriostoma discors in general appearance, 
the present species differs from it in the presence of only one 
keel on the body-whorl. This, on the inner whorls is sutural, 
instead of being median. The ornamentation is also very 
different, that of 0 . discors , consisting of close, thin, anasto- 
mosing lamellae, in place of the previously described costse of 
0. northi . 
The groups of shells so characteristic of the Wenlock 
strata, to which 0. northi belongs has not, to my knowledge, 
been before described from Australian Silurian rocks, although 
a note attached to one of the Quarter Sheets of the Victorian 
Geological Survey,* published many years ago, records the 
occurrence of a somewhat allied shell, Oriostoma sculptum , J. 
ue C. Sby., sp., in the neighbourhood of Kilmore. 
One word on the genus. According to M. JMunier Chalmas’ 
original definition, the spire is said to be an uncoiled one, and the 
whorls free. On the other hand l)r. Lindstrom says, “ whorls 
. . . . joined, seldom a little disjointed near the aperture.” 
It seems to me that a more particularised diagnosis than this 
would be advantageous. Dr. Paul Fischer j goes even further 
and says the whorls are contiguous. 
* Quarter Sheet 4, S.W. Geol. Survey Victoria, (Note B620). 
fMan. Conohyl. 1887, p. 813. 
