158 GENESIS OF THE ARIETIDA. 
These at first have the depressed aspect of the adult pilee 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 pile 
acquire immature geniculea. The channels make their appearance upon the 
second quarter of the fourth whorl, and the bent genicule 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 suleations deepen and broaden, and the genicule 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 spiralissimum 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 smratissinum. Five volutions of spératissimum have about the same diam- 
eter as four or four and a half of Conybeari. There are about twenty-five pile 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 pila more salient, and the whorl 
arrower 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 spiralissmum. 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 Zodlogy 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 
“TLias Amm.,” 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 Zoilogy, 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 Tiibingen is a specimen, perhaps the same figured by 
Quenstedt, Plate XV. Fig. 1, with undoubted spines on the casts of the genicule. 
The original of Sowerby’s figure is 455 mm. in diameter, and had begun to lose 
