158 GENESIS OF THE ARIETID^E. 



These at first have the depressed aspect of the adult pila? in Caloceras, and the 

 rotundity of the abdomen increases the resemblance to this genus. On the first 

 quarter of the fourth whorl the abdomen grows broader, flatter, and the pilce 

 acquire immature geniculoe. The channels make their appearance upon the 

 second quarter of the fourth whorl, and the bent geniculse of the adult become 

 apparent also, though very obscure. When the lateral ridges of the sulcations 

 are developed on the fifth whorl, the shell becomes similar to Ver. spiratissimum ; 

 and finally, as the sulcations deepen and broaden, and the geniculae become more 

 salient and bend more forward, the adult characteristics of the species are fully 

 brought out. 



The young as compared with the young of spiratissimum present considerable 

 differences. They are broader in proportion and increase faster in bulk. Thus 

 they form an umbilicus deeper and with fewer volutions within a given diameter 

 than in spiratissimum. Five volutions of spiratissimum have about the same diam- 

 eter as four or four and a half of Conyheari. There are about twenty-five pilce on 

 the third whorl, thirty-six on the fifth or sixth whorl, and forty on the seventh 

 whorl. 



The channels are deeper and broader, the lateral ridges and keel sharper and 

 narrower, the sides more deeply furrowed, the pike more salient, and the whorl 

 narrower from side to side in proportion to the breadth of the channel area, and 

 narrower also in proportion to the dorso-abdominal diameter than in spiratissimum. 

 The sutures also differ considerably. The abdominal lobe in some specimens 

 is one half deeper than the superior laterals ; the inferior lateral saddles are 

 deeper than the superior laterals, and the inferior lateral lobes shorter than the 

 superior laterals ; the auxiliary lobes and saddles continue the inclined line formed 

 by the apices of the lobes. This anterior inclination is subject to variations in 

 the adults of varieties. In the young the lobes and saddles are nearer to the same 

 level, and approximate to the outlines of the sutures in spiratissimum. In Pro- 

 fessor Fraas's collection there are two specimens, one in which the superior and 

 inferior lateral lobes are about equal, and one in which the inferior laterals are 

 a little the longer ; these are both full grown. 



A specimen in the Museum of Stuttgardt has a living chamber nearly one 

 and a half volutions in length, and still incomplete. 



One specimen in the Museum of Comparative Zoology completes nine and 

 one half whorls without exhibiting any senile characteristics. The largest speci- 

 men yet recorded is now in the collection of the British Museum ; this measured, 

 according to Wright, about 460 mm. in diameter. The one figured by Wright, in 

 " Lias Aram.," was 340 mm. in diameter, and old age had begun to show its effect 

 slightly upon the last whorl. A specimen from Lyme Regis, in the collection of 

 the Museum of Comparative Zoology, is associated upon the same slab with 

 Birchii. The largest specimen in the Stuttgardt Museum was 365 mm. in diameter, 

 and had not yet begun to exhibit very decided senile characteristics. 



In the collection at Tubingen is a specimen, perhaps the same figured by 

 Quenstedt, Plate XV. Fig. 1, with undoubted spines on the casts of the geniculte. 

 The original of Sowerby's figure is 455 mm. in diameter, and had begun to lose 



