138 : GENESIS OF THE ARIETIDA. 
of the keelless Johnston’, but gradually become elevated a little on the abdomen, 
and at the same time acquire a faint keel. Subsequently the abdomen becomes 
more elevated, and slight channel-like inflections appear on either side. At the 
diameter of 98 mm. this occurred in one specimen from Beauregard in the collec- 
tion at Semur. At the diameter of 195 mm. in other specimens from Beauregard, 
the faint channels had disappeared, the pile: were nearly obsolete, and the keel 
not so distinct. The marginal digitations of the lobes and saddles were also 
deeper and much changed. At the diameter of 270 mm. senile changes were far 
advanced in the only specimen of this size yet studied. The abdomen had 
become rounded and keelless, and the pile so nearly obsolescent as to be barely 
distinguishable. 
A specimen from Beauregard in the collection of the Sorbonne, under the 
name of Anum. laqueolus, Schlénbach, must have been when complete about 200 mm. 
in diameter. The outer whorl of this shell resembles a typical ¢ortile, with 
depressed abdomen and keel. The abdomen at earlier stages in the same speci- 
men is rounded. Laqueolus, Schlonbach, is not of this species, but belongs to 
Cal. Liasicum. 
In some specimens the abdomen changes from rounded and rather flattened 
in the young to a more angular outline in older stages, but does not acquire a 
keel. This occurs'in D’Orbigny’s original at the Ecole des Mines, and in speci- 
mens labelled fortilis from Chalandry, in the collection of the Sorbonne, at sizes 
varying from 22 to 55 mm. _ It is probable that a well defined keel never made 
its appearance even in the adults in some of these specimens. 
Var. raricostatoides. 
There are two specimens in Quenstedt’s collection from Quedlinburg, three 
in the Museum of Comparative Zodlogy, and others at Semur, all found in 
association with Planorbis. 'These seem to be identical with the forms described 
by Dunker in the “ Paleontographica” as Amm. raricostatus. 
A part of the specimens identified by Oppel as Johnstom in the Munich col- 
lection appear to belong to Cul. tortile. One specimen especially is a rather 
compressed form, from Waldenburg, with a slight keel developed at a late stage 
of growth, and at the same time there is a change of form in the whorl, which 
approximates to the parallel-sided, flattened-abdomened, carusense-like varieties 
of Cal. Nodotianum ; the pile also begin to wear a more advanced aspect. 
The torus-like variety is finely represented in the Bristol Museum by speci- 
mens from Cotham, and a form which appeared to be the same was reported as 
coming from the Bucklandi bed at Ashley Down. This has somewhat. stouter 
whorls than the earlier forms at the same age, and shows the same tendency 
to become stouter and larger manifested in the later occurring species of other 
progressive series. The young have the usual development of forti/e, in some 
instances producing a decided keel, and in others merely a slightly more com- 
pressed form in the adult or old, the abdomen remaining keelless. They have 
straight pile, and look remotely like raricostatus in form, but the young have 
