236 



inteniipted, terminating at the outer carinse; back rounded, witli a large 

 prominent obtuse keel, bounded by two deep sulci and two external 

 lateral carinse ; aperture oblong, nearly quadrate. 



Dimensions. — Transverse diameter, 10^ inches ; height of ajjei-ture, 2 

 inches ; width of aperture, 2 inches. Specimens measured by M. Hauer, 

 3 J inches diameter; height of the last whorl, yV^; width, ■^■^; diameter 

 of the umbilicus, —^ of the diameter of the shell. 



Description. — This is one of the most characteristic and widely dis- 

 tributed species of the zone of Ammonites Bucklandi, as shown by the 

 table of synonyms prefixed to this article. Its form in general is very 

 persistent, and its specific characters are subject to fewer variations 

 than most of its congeneric forms. The specimen I have figured was 

 obtained from the same locality as the type drawn by Sowerby, namely, 

 the Bucklandi beds of "Weston, near Bath, from cuttings made during 

 the formation of the Great Western Railway. Another very fine typical 

 example, collected at the same time and from the same locality, is con- 

 tained in the British Museum. "It measures 18 inches in greatest 

 diameter, and 4 inches across its last chamber in thickness; eight whorls 

 are capable of being coiinted, but the innermost jjortion is not capable of 

 being made out." Mr. Henry Woodward, F.G.S., has kindly given me 

 these measurements. 



The adiilt shell consists of from eight to ten slightly involute whorls, 

 about the width of their height ; the inner whorls are nearly all entirely 

 exposed, the turn of the spire only concealing a portion of the back. 

 The sides are convex and costated. The ribs are simple, narrow, obtuse, 

 and slightly arched, and disappear at the outside of the lateral carinse, 

 near the dorsal sulci ; they are numerous, and increase with the diameter 

 of the shell ; in the specimen I have figured there are 68 ribs in the 

 outer whorl ; in general terms the number may be said to range from 

 40 to 60. 



The back is broad and provided with a lai'ge obtuse prominent keel, 

 on each side thereof is a deep sulcus bounded by an outer carina, so that 

 the back of this shell is tricarinated with two sulci. 



The spire is composed of whorls, with convex costated sides nearly 

 fully exposed, the outer whorl having a strongly developed tricarinate 

 back. 



Mouth apertvire, neai'ly quadrate with rounded angles. The septse, 

 according to d'ORBiGNY, are symmetrical, foliated on each side, and divided 

 into two or three lobes formed of nearly equal branches. Dorsal lobe 

 much longer and as wide as the superior lateral lobe, formed of a single 



