19/3 TERMINOLOGY ix 



For dimensions exceeding 50% of the diameter, = 100% of the 

 radius approximately, the term extremi- can be affixed, modified up 

 to 66% by sub-, from 83% by per-. 



Thus the figures given above (p. viii) namely 22, 22, 55, show a species 

 substeno-, subleptogyral, subextremi-latumbilicate. 



Taking the radius as 50% of the diameter will make very little 

 difference in the case of polygyral shells, but it will make some difference 

 in the case of oligogyral specimens : those which have been described 

 on the actual radius will sometimes shew discrepancies when described 

 on half the diameter. But the conveniences of the present plan far 

 outweigh any disadvantages in such cases. 



Ammonite Development 

 See Vol. I, p. x. 



Some additional terms are required : — 



Cadicone (cadus, a cask), coiled cones with divergent sides and 

 broad venter. Examples : Cadoceras, Erymnoceras, Teloceras, Ccdoceras, 

 Emileia crater sp. n. 1 Tropites, Gastrioceras , etc. 



It is a transitional stage which, by umbilical contraction, may pass 

 to sphserocone (Cadoceras), or by umbilical expansion and whorl com- 

 pression may produce serpenticones (Skirroceras, ex Teloceratan form) ; 

 or the sequence may be cadicone, sphserocone, serpenticone (Emileia). 



Platycone, coiled cones with broad, flat sides which are not 

 oxycones, because they have not acute venters. The stage is transitional 

 between serpenticone and oxycone, and has sometimes been expressed 

 in earlier parts of this work as " becoming oxycone." Examples : — 

 more or less platygyral species, especially when also more or less lepto- 

 gyral : — Arietites, Hildoceratids, Witchellia. 



1 This remarkable species, an Emileia in the cadicone stage, has a Teloceratan 

 aspect with a deep craterumbilicus, though the body-whorl is contracting, as is 

 the rule in all old-aged specimens of Emileia. The proportions of the type are : — 

 ^3, 34» 47. 4° 



I2 4> 36. 67, 34 



io 3. 36, 74. 34 



Thus the conch contracts 7% in the first half of the last whorl, and 20% in the 

 second half, while the umbilicus expands 6% in the last half-whorl. So the species 

 is platygyral, extremi- to pachygyral, latumbilicate. 



This species is a most interesting link connecting Emileia with an unknown 

 Teloceratan ancestor, for it shows that the genus begins in the cadicone (Teloceratan) 

 stage. Club-shaped costae instead of spines distinguished it from Teloceras, so does 

 a more complicated suture-line. 



On the evidence of this species Emileia was placed as springing from Cceloceras 

 (now Teloceras) in Jur. Time; Quart. Journ. Geol. Soc, 1898, LIV, Table 11. 



The type is from the Bajocian, [Shirbuirnia zone], Sandford Lane, Sherborne, 

 Dorset. No. 1242, S. Buckman Coll. 



