AEGOCERAS DAVCEI. 347 



here figured formerly belonged to my late friend, Mr. John Leckenby, F.G.S., who 

 obtained it from Charmouth ; it is one of the finest examples known, and is now 

 in the Woodwardian Museum, Cambridge ; the shell is discoidal and compressed, 

 consisting of rounded whorls, which are only slightly involute, and closely clasp 

 the penultimate volution ; they are covered on the sides and area with a series of 

 ribs and sulcations about equal in width throughout ; on the sides the ribs are directed 

 obliquely forwards, whilst across the siphonal area they are transverse and extremely 

 regular. In typical shells each whorl develops from eight to twelve round blunt 

 tubercles about the middle of the whorl, which occupy the space of two ribs and 

 two sulcations ; sometimes these tubercles have a thick envelope, and then they form 

 spines ; many specimens, however, are wanting in the tubercles, though in the figured 

 specimen the projections are large and regular, and impart a very distinctive 

 character to this Ammonite. The lobe-line is very complicated ; the siphonal lobe, as 

 long and wide as the principal lateral, is formed of three branches, two of which 

 bifurcate. The siphonal saddle, as large as the principal lateral, is very irregularly 

 divided into three folioles at the external side, and one at the internal. The principal 

 lateral lobe divides into two large branches, the terminal one is very much ramified, and 

 the external, nearly as large, is equally ramified. The lateral saddle is smaller than the 

 principal lateral lobe, and terminates in four folioles. The lower lateral lobe is small, 

 with five digitations, and the auxiliary lobe, still smaller, possesses three. The lobe-line 

 has been extremely well figured by Quenstedt and d'Orbigny ; none of the specimens 

 which have passed through my hands show this structure so well, as they have all retained 

 their shell (or portions thereof) which conceals the true septal suture. 



Prof. d'Orbigny had the opportunity for studying the evolution of this Ammonite ; 

 he observes :^ — " This species, more or less compressed, varies much according to age. 

 Up to a diameter of 25 millimetres the whorls are much depressed, and ornamented 

 with from thirteen to fifteen long, sharp-pointed spines ; this depression of the whorls is 

 often remarked up to a diameter of 50 millimetres ; beyond this diameter there are in 

 general from eight to twelve tubercles ; the whorls become more or less thick and com- 

 pressed. At the largest diameter known (1 20 millimetres) the ribs become more prominent, 

 and more irregular, and the tubercles are less regularly placed. The spines appertaining 

 to the shell leave a truncated tubercle on the mould." 



Affinities and Differences. — This species resembles Aeg. Lechenhyi in its ribs and 

 spines, but is distinguished from that form by its tubercles being set much wider 

 apart, by its regular oblique narrow ribs and valleys on the sides, and by the finer 

 sculpture across the area ; the structure of the lobe-line likewise presents an important 

 difference between the two species. 



Locality and Stratiyraphical Position. — This is a capital leading fossil for a certain 

 horizon of the Middle Lias ; the chief locality in England is the Green Ammonite-beds 

 1 ' Paleontologie Fran9ai8e ; Terr. Jurrassique,' t. i, p. 277. 



