MULTILOCULAR SHELLS. 253 



ribs, which are numerous towards the back, but fewer and more 

 prominent towards the inner side of the whirl. The aperture ap' 

 proaches to ovate. Sowerby's a. Calloviensis, Tab. 104, Fig. 3, and 

 a. Duncani, Tab. 157, resemble this shell ; but as they both differ 

 from it, we may name it a. Rowlstonensis. A beautiful nacrous shell, 

 not unlike this species, occurs in the upper shale; but we have no 

 complete specimen. — No. 11, PI. XIII, is another small, beautiful, and 

 rare ammonite, from the beds corresponding with the Staiths band in 

 the front of Cranimoor. It resembles a. Maltonensis, both in its 

 shape and its markings ; but its ribs are waved, and are less prominent, 

 and extend across the whirl to near the crenulated keel, where they 

 bifurcate. We may name this shell a. Clevelandicus. 



A very large globose smooth ammonite, with a deep cavity or 

 umbilicus on each side, occurs in the Malton oolite, which, from the 

 great size of the cavity, may be named a. capax. Sowerby's a. 

 sublcEvis, Tab. 54, resembles it, but is not the same species. — A few 

 more species of the ammonite family are described in the Scarborough 

 Catalogue. 



Nautilus. The difference between the ammonite and the nau^ 

 tilite is merely this, that in the former all the whirls are exposed, 

 while in the latter the outer whirl conceals all the I'est. We have 

 dismissed the ill defined genus ammonoides, under which some au* 

 thors include part of the amnlonites last described; and we have 

 contented ourselves with naming the proposed genus ammonautilus, 

 intermediate between the ammonite and nautilite; because it would 

 be difficult to fix the limits of such a genus, there are so many grada- 

 tions between those species in which the whirls are fully exposed, 

 and those in which they are almost entirely concealed. In like 

 manner, the transition from the latter to the urabilicated nautilus is 

 so shortjthat there is perhaps an impropriety in making even the 

 nautilites a distinct family. We may notice, hoAvever, that in general 



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