322 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [April 9, 



cated at the upper third of the whirl ; the ribs and their bifurca- 

 tions are all bent gently forwards, and each whirl exhibits three or 

 four contractions at nearly regular intei*vals of growth. Spire 

 formed of seven or ten whirls, according to age, which are all well 

 exposed ; their greatest diameter is near the umbilical border ; from 

 this point the sides slope gently towards the dorsal border. Back, 

 in middle age, narrow, rounded, and smooth both in the shell and 

 mould. Mouth oblong, compressed on the sides, and becoming nar- 

 row on the dorsal third ; when entire, it terminates in two /-shaped 

 processes. Umbilicus large, with stair-like sides ; the whirls em- 

 bracing the half-smooth part of the shell, and exposing the inner 

 ribbed portions thereof. 



The septa are symmetrical on the dorsal and lateral parts of the 

 whirls, and consist of a dorsal lobe and five lateral lobes ; the 

 dorsal lobe is much larger than, but not so long as, the superior lateral 

 lobe ; it is divided by the median line into two parts for about one- 

 third of its length, aud is formed of six symmetrical, nearly equal- 

 sized lobules, which are provided with three or four unequal digitate 

 margins ; the superior lateral lobe is smaller, but much longer than 

 the dorsal ; it is formed of eight unsymmetrical unequal-sized lobules ; 

 those on the upper side are more developed than those on the under 

 side, and the terminal leaf is much the largest, with large lateral 

 bifid ramifications ; the inferior lateral lobe is small, and directed 

 obliquely backwards ; it is composed of two lateral lobules, and one 

 terminal lobule, which are unequal in size and either trifid or quad- 

 rifid on the margins ; the three ventral lobes decrease in size from 

 above downwards; they are all directed obliquely upwards; the upper- 

 most lobe is tridigite, the second is bidigite, and the third consists 

 of a single lobule with jagged margins. 



Up to the diameter of 6 or 8 inches the shell retains its ribbed 

 character, as described ; but when it grows beyond that size, it loses 

 all its ribs, the whirls become more thick, the back more rounded, 

 the shell smooth, and only marked with lines of growth and slight 

 periodical constrictions. 



Dimensions. — Usual size from 8 to 9 inches in diameter ; the 

 largest specimen known is in the British Museum, and measures 

 16 inches. 



Affinities and differences. — This species, up to the diameter of 

 6 inches, resembles Ammonites ParJdnsoni, and is fiequently named 

 as such in public collections : this may have arisen in part from the 

 remark made by Sowerby in his description of Ammonites Parkin- 

 soni, that " it is the Ammonite so frequently split, polished, and 

 sold at Bath ; " and again, " I suspect it may also be found in the 

 lower beds of the Iron-shot Oolite, as the specimen figured is from 

 near Yeovil." Now the fact is, that Ammonites Parkinsoni so well 

 figured by Sowerby (Min. Conch, tab. 307. fig. 1) rarely exceeds 

 6 or 7 inches in diameter, and is not often either split or polished ; 

 whereas the specimens of Ammonites Dorse tensis selected by " the 

 gothic hands of the mason," almost always exceed these dimensions. 

 AH the largest and finest specimens of Ammonites Parkinsoni which 



