SOME NEW SPECIES OF AMMONITES. 139 



slightly produced on the ventral area ; mouth-border perfectly 

 plain, slightly produced on the ventral and dorsal areas ; keel 

 very small, barely distinct ; inner portion of whorls very convex 

 and sloping ; body-chamber about two-thirds of a whorl in 

 length. 



This peculiar and distinct species sometimes attains a fine size, 

 about 12 to 15 inches, perhaps more, across. "When adult its 

 characters are much changed. The umbilicus becomes wider, 

 and the shell much flatter in proportion. Many fine specimens 

 are in the collections of Mr. Darell Stephens, Mr. T. C. Maggs, 

 and others, obtained from near Sherborne, Dorset. The quarry 

 is now, unfortunately, partially closed, and this species has not 

 been met with for some years. The specimen figured was 

 collected by my father. It is the only one I have seen showing 

 the mouth border. The figure is two-thirds the natural size of 

 the original. This species is named in compliment to Darell 

 Stephens, Esq., F.Gr.S., &c. 



Locality, near Sherborne, Dorset. 



Position, probably zone of Harp. Sowerbyi. 



The suture line seems rather simple. Owing to the very good 

 preservation of the specimens I have not seen the sutures 

 properly. 



I am not certain of the genus of this species. It has close 

 affinities with Ammonites Truellii, d'Orb and ammonites fisilolatus, 

 Waagen. In my former paper following Bayle I put Ammonites 

 Truellii in the genus Oppelia, and I described Ammonites 

 fissilolatus as Harpoceras. Since then, however, I have observed 

 that Am. jissilolatus, though resembling Harpoceras rather 

 closely, differs from it on account of its very complicated suture- 

 line, while Am. Truelli differs from Oppelia on account of its 

 large, very distinct keel, and generally greater thickness. These 

 three species and Oppelia sulcostata (J. Buckman) and another 

 species from the Inferior Oolite, undescribed as yet, seem to 

 form at leabt a very distinct group, if not distinct genus. 



