138 GENESIS OF THE ARIETID.E. 



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 pilas 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 pilce so nearly obsolescent as to be barely 

 distinguishable. 



A specimen from Beauregard in the collection of the Sorbonne, under the 

 name of Amm. laqueolus, Schlonbach, must have been when complete about 200 mm. 

 in diameter. The outer whorl of this shell resembles a typical tortile, 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. Liuslcum. 



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 tortilis 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. 



Vak. raricostatoides. 



There are two specimens in Quenstedt's collection from Quedlinburg, three 

 in the Museum of Comparative Zoology, 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. raricostatm. 



A part of the specimens identified by Oppel as Johnstoni in the Munich col- 

 lection appear to belong to Cal. 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 Cat. Nodotianum ; the pilse also begin to wear a more advanced aspect. 



The torus-like variety is finely represented in the Bristol Museum by speci- 

 mens from Cutham, 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 tortile, 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 pilae, and look remotely like raricostatus in form, but the young have 



