65 



keel is continuous, forming the 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, w^ith an ill-defined spiral obtuse keel. Upper surface 

 of the w^horls 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. 



Ohs. — In mature individuals the back broadens, the sides of 

 the body w^horl become more rounded, the costse towards the 

 aperture broaden out into folds, and the whole shell puts on a 

 nautiloid appearance. Furthermore the costfB on the lower 

 surface become obliterated. These changes have been alreadj'^ 

 noticed by Dr. Liudstrom in Oriostoma discors, of which he 

 remarks that " 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." Tlie costal 

 bifurcate both at the suture and the bounding keel, and are as 

 a rule about their own distance apart, but in j^assing 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 w^horls is sutural, 

 instead of being median. The ornamentation is also very 

 different, that of 0. discors, consisting of close, thin, anasto- 

 mising lamellae, in place of the previously described costaj of 

 0. iiortlii. 



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. 

 cte C. Sby,, sp., in the neighbourhood of Kilmore. 



One word on the genus. According to M. Munier Chalmas' 

 original definition, the spire is said to be an uncoiled one, and the 

 whorls free. On the other hand Dr. 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 f goes even further 

 and says the whorls are contiguous, 



* Qiiarter Sheet 4. S. W. Geol. Survey Victoria, (Note B620). 

 t Man. Conchyl. 1887, p. 813. 



