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INFERIOR OOLITE AMMONITES. 



it differs by more regular, less profuse ornamentation, and a wider umbilicus ; from 

 acanthocles and crassispinata a and /3 by more flatly-sided whorls, especially in the 

 umbilicus, and by less ornament ; while it is only to crassispinata y that it bears 

 any great likeness. From that form it is separated by its less reclining ribs, its 

 spines more marked in the umbilicus and less marked on the outer whorls, its 

 slower coiling, the want, for the most part, of the space between the spines and 

 the inner margin, and the absence of that declivity in the same place which is so 

 marked in crassispinata, acanthocles, &c. 



S. spinicostata is not the adult of multispinata ; sufficient of the inner whorls, 

 with their flat sides and irregularly-placed coarse ribs, is left to show that. Its 

 inner whorls are the morphological representation of a flat-sided adult form orna- 

 mented like crassispinata a, of which such form may have been the cousin. 



The specimen figured is the only example in my collection, and it came from 

 the Co?icavnm-zone, Bradford Abbas. Its side view is depicted two-thirds the 

 natural size in PI. LXXIII, fig. 4, and an outline of the aperture similarly reduced 

 in fig. 5. The suture, fig. 6, is of the natural size. 



Sonninia costata, 8. Buchnan. Plate LXXIV, fig. 1 ; Plate LXXV, figs. 1, 2. 



Discoidal, compressed, cariuate. Whorls, in section, compressed, oblong, 

 ornamented only with direct, nearly upright costas, inclined but obscure ventrally. 

 Ventral area flattened, fairly defined, possessing a small solid carina, bordered by 

 very obscure sulci. Inner margin well-marked, slightly sloped, the upper edge 

 rather angular. Inclusion about one-third. Umbilicus concentric and regularly 

 graduated. Suture-line with a broad -stemmed superior lateral lobe, of which the 

 lateral lobules are unimportant. Termination of adult evidently perfectly plain, 

 parallel with the ribs and growth-lines. 



The ribs of this species are, in size, rather irregular. There is sometimes 

 more or less of a tendency to produce rather coarse, more distant ribs in the adult, 

 as in the figured example ; in other specimens this is not so noticeable, and the 

 ribs tend to be obsolete in old age. The ribs are rather obscure on the ventral 

 area, and in the forms which connect the species with the next (parvicostata) they 

 tend to be obscure on the outer third of the whorl as well, though fairly marked 

 and distant on the inner two-thirds. 



Besides the examples which connect this and the next species, I have 

 distinguished two principal forms as costata : 



a. The type, thin and umbilicate (PI. LXXIV, fig. 1). 



/3. Thicker, less umbilicate. 



The flattened sides and ventral area, and the prominent inner margin give to 



