AEGOCERAS ANGULATUM. 319 



Septa symmetrical (PI. XVll, fig. 1), divided into five lobes and six saddles, formed 

 of unequal parts ; the siphonal lobe much shorter and broader than the principal lateral 

 lobe, divided half its length by the median line into two parts, it is formed of two or 

 three short branches on each side, and has a terminal trifid digit ; the siphonal saddle 

 is much larger than the principal lateral lobe, and terminates in three festoons, which 

 divide into folioles ; the principal 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 principal 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 auxiliary lobes are placed very obliquely, and gradually diminish in size as they 

 approach the umbilical angle. 



In my large specimen the body-chamber consists of four-fifths of the last whorl, its 

 sides are smooth, and slope towards the outer border, which is roxxnd and narrow, without 

 the trace of a keel ; there are from thirty to forty long, faint, biflexed ribs, extending from 

 near the umbilical to the outer margin, and shorter intermediate costse spring from the 

 bifurcation of the former. All the ribs become much more developed as they pass round 

 the siphonal border, before they suddenly disappear. 



Affinities and Differences. — This Ammonite belongs to the group Angulati, and to 

 the genus Aegoceras ; in early life its strong, sharp ribs form an angle on the siphonal 

 area (fig. 6). The marginal furrow which interrupts them (fig. 4) likens it to some Gault 

 species ; but adult shells can scarcely be mistaken for any other form associated with 

 them in the Lower Lias (figs. 4 — 6). 



This Ammonite presents many varieties of form during different stages of growth, 

 which have been mistaken for distinct species ; it is now ascertained that Aego- 

 ceras Moreanum, and Aeg. Leigneletii, d'Orbigny, are different varieties of Aeg. 

 angulatum. 



Locality and Stratigraphical Position. — I collected this Ammonite between Charmouth 

 and Lyme Regis, in a dark shale below the grey concretionary limestone with a 

 mamillated surface, which forms the base of the BucMandi-heds, and in the same 

 stratum south-west of the Cob ; the zone which it characterises is of no great 

 thickness in Dorsetshire. This Ammonite is considered one of the rarest found on 

 that coast. In Gloucestershire I have collected small ribbed specimens from the lowest 

 shales of the Fretherne section on the banks of the Severn, and it has likewise been 

 found at Aust Cliff. In Warwickshire some fine specimens were obtained from the 

 Harbury cutting of the Great Western Railway, near Southam, and many good fragments 

 are still to be found on the spoil banks of that line. In Yorkshire it is found only in 

 the lowest beds of Lias at Redcar, where it is very rare, and known to local collectors as 

 Am. Eedcarensis. It is found in the same zone of Lower Lias near Portrush, on the 

 north coast of Ireland. 



