188 GENESIS OF THE ARIETIDA. 
rotiforme may be observed by comparing the figures on Plates VII. and III., but 
from these it may be distinguished by the flatness of the abdomen, the elevated 
geniculee, and the much earlier development of the channels. 
At Semur there are several varieties of this species. One was 620 mm. in 
diameter, the last whorl about 170 mm. in diameter from abdomen to dorsum. 
The abdomen was quite narrow on the last whorl, but the tubercles were still 
traceable, or rather the genicula were very thick and prominent, resembling 
tuberculations. Another specimen 650 mm. in diameter, has a last whorl 240 mm. | 
in diameter, and the abdomen much narrower, about 110 mm. in breadth, 
the dorsum being fully 160 mm. in breadth. The keel in this more advanced _ . | 
| ‘senile stage is considerably reduced in size, and the channels are almost obso- 
lete ; the tubercles are nearly obsolescent, but the genicule still rise above the 
edge of the abdomen. ‘This peculiarity of the pile shows that the Sauzeanus 
form is persistent, and it enables the observer to distinguish the differences 
between a member of this series and all others. 
Wright’s figure’ of a specimen from Semur, named by him Arietiles rotiformis, 
looks like the full grown adults of the more discoidal variety of this species, but 
his reference of it to the Lower Bucklandi bed is probably incorrect. Dumor- 
tier’ declares that a species identical with disudcatus, D’Orb., but with channels 
not so deep, is found in several localities in the Angulatus bed in the basin of 
| the Rhone. Unfortunately, no figures accompany his description, and figures 
of young and adult would be necessary to support this opinion. Cor. kridion 
and Verm. Conybeari are very like disulcatum in the adults, but are quite distinct 
at earlier stages. 
We think perhaps the more involute and broad whorled variety of this species 
should be recognized as distinct. It is the Amm. resurgens of Dumortier, and the 
subnodosus of Wright. Wright was mistaken in placing his specimen with Amal- 
theus, it being an undeniable Arietian, probably occurring not later than the 
| Obtusus bed, and perhaps as early as the Upper Bucklandi bed. In the Museum 
of Comparative Zodlogy there is also a magnificent specimen from Lyme Regis, 
measuring 375mm. This is a perfect cast of one side, showing the same char- 
acters as in the small specimens figured by Dumortier, and the one figured by 
Zieten as multicostatus. These characters persist even at that advanced stage, 
without any signs of senility, and the pile, tubercles, genicule, and form of 
whorl remain unchanged. 
The examination of Sowerby’s original has induced us to join Oppel in sup- | 
pressing the name of multicostatus. Sowerby’s fossil is precisely similar to the 
adults of the common French variety of this species. 
a. 
1 Lias Amm., pl. ix. ij 
2 Htudes Pal. du Bassin du Rhone, pt. 1, p. 115, and pt. 2, p. 115. 5 
8 Lias Amm., pl. vi. fig. 2, 3. [ 
