171 



period the ribs divide, become less distinct, and the apgle on the back 

 disappears ; in advanced age the sides are smooth, the back narrow, and 

 without a keel or ribs. 



Septa symmetrical (fig. 4 b) divided into five lobes, and six saddles 

 formed of unequal parts ; the dorsal lobe much shorter and broader than 

 the superior lateral lobe, divided into two parts to half its length by the 

 median line, and formed of two or three short branches on each side, and 

 a terminal trifid digit. The dorsal saddle is much larger than the 

 superior lateral lobe, and terminates in three festoons which divide into 

 folioles. The superior lateral lobe has four small lateral digitations, and 

 a terminal digit, which divides into three or four branches. The lateral 

 saddle, about as large as the superior lateral lobe, terminates in one 

 central, and two lateral festoons, the former with four, the latter with 

 three smaller foliations. The inferior lateral lobe is oblique, with three 

 or four lateral, and a terminal, sometimes bifurcate, digit. The three 

 avaciliary lobes are placed very obliquely, and gradually diminish in size 

 as they approach the umbilical angle. 



In my large specimen the dwelling chamber consists of four-fifths of 

 the last whorl, its sides are smooth, and slope towards the back, which is 

 round and narrow, without the trace of a keel ; there are from thirty to 

 forty long faint biflexed ribs extending from near the suture to the back, 

 and shorter intermediate ones springing from the bifurcation of the 

 former. All the ribs become much stronger marked as they pass round 

 the dorsal border, where they suddenly disappear. 



Affinities and Differences. — This Ammonite belongs to the gi-oup 

 Arietes, and to the section without a keel ; in early life its strong, sharp 

 ribs form an angle on the back, and the doi-sal furrow which interrupts 

 them (fig. 4 a) liken it to some Gault species, but adult shells can 

 scarcely be mistaken for any other form associated with them in the 

 Lower Lias (fig. 5.) 



This Ammonite presents many varieties of form during different stages 

 of gi'owth, which, when studied per se, have been mistaken for distinct 

 species, for it is now ascertained that Ammonites Catenatus, A. Moreanus, 

 A. GlMrmassei, A. Ldgneletii, of d'OKBiGNY, are all different varieties of 

 A. Angidatu^. 



Locality/ and Stratigraphical Position. — I collected this species between 

 Chai-mouth and Lyme Regis, in a dark shale below the gi-ay concretionary 

 limestone with a mammilated surface, which forms the base of the 

 Ammonites Biicklandi beds, and in the same strata south-west of the 

 Cob, The zone which it characterizes is of no great thickness in 

 Dorsetshire, and this Ammonite is considered one of the rarest found on 



