[88 GENESIS OF THE ARIETID.E. 



rotifornie 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 

 geniculae, 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 geniculae 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 1G0 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 geniculae still rise above the 

 edge of the abdomen. This peculiarity of the pila? 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 1 of a specimen from Semur, named by him Arietites rotiformk, 

 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 2 declares that a species identical with bisulcatvs, 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. Conybean are very like bisulcaium 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: 5 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 Zoology there is also a magnificent specimen from Lyme Regis, 

 measuring 375 mm. 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 muMcosicdus. These characters persist even at that advanced stage, 

 without any signs of senility, and the pilae, tubercles, geniculte, and form of 

 whorl remain unchanged. 



The examination of Sowerby's original has induced us to join Oppel in sup- 

 pressing the name of mvttieostatus. Sowerby's fossil is precisely similar to the 

 adults of the common French variety of this species. 



1 Lias Amm., pi. ix. 



2 Etudes Pal. du Bassin du Rhone, pt. 1. p. 115. and pt. 2, p. 115. 



3 Lias Amm., pi. vi. fig. 2, 3. 



