LIOCERAS. 21 



Hildoceeatinj:. 



Genus — Lioceras, Hyatt, 1867. 



{Type— Lioceras OPALi>*iTM, Eeinecke, sp.) 



Definition. — Compressed, discoidal, divisible into two groups, one, tlie more 

 typical, subcarinated, the other acutely and distinctly carinated. Umbilicus in 

 general small, the inner whorls in some species being almost entirely occluded. 

 Inner margin always concave. Termination, so far as known, has a double bend, 

 sometimes produced and pointed on ventral area. 1 Body-chamber just half a 

 whorl. In young state ornamented with falciform, more or less bifurcating ribs, 

 in adult with plain falciform ribs 2 inconspicuous on inner area or else the whole 

 perfectly smooth, with merely lines of growth. The inner margin of body- 

 chamber always recedes from the regular line of coil of the centre, giving 

 to the umbilicus of complete specimens a sub-oval appearance. As the shell 

 grows and the chambers are formed, the inner margin advances towards its 

 regular place nearer the centre. (This peculiarity exists in some genera of 

 other groups of Inferior Oolite Ammonites and requires to be well understood.) 

 The suture line is peculiar and very constant. The general characters are that 

 the siphonal lobe is rather short. The siphonal saddle is divided into two 

 parts by a well-marked accessory lobe, the inner part being invariably the 

 deeper. The principal lateral lobe is broadly stemmed, digitated, and with three 

 terminal branches which partly penetrate the same lobe of the preceding suture 

 line and almost touch the preceding saddles on both sides of it. The lateral 

 saddle is divided by an accessory lobe. The inferior lateral lobe is smaller than, 

 but very similar to, the principal one ; there are four auxiliary lobes and three 

 saddles, the first lobe being well developed. 



Remarks. — This genus, as I have pointed out, is divisible into two portions, the 

 one almost without a distinct keel, the other with a very distinct, prominent keel, 

 leaving the ventral area flat and almost furrowed. The principal lateral lobe of this 

 latter class has not three such distinct terminal branches, but I am unable to say 



1 The young specimens of some, species of this genus have lateral lappets. Lioceras opalinum 

 also possesses these lappets at maturity. 



2 The character of the ribs in adult Lioceras is shown on Plates V and VI. The inner area 

 is smooth and depressed, the secondary ribs on the outer area are few but conspicuous, and appear 

 like a succession of notches. Still the type of the genus, Lioceras opalinum, has really no ribs, 

 but merely very numerous lines of growth, which are sometimes fasciated (as Dr. Wright calls it), 

 producing false irregular ribs. 



