138 



THE GEOLOGIST. 



angle, either provided or not with salient tubercles. Back salient, keeled^ 

 and containing the syphon. Mouth in the perfect state prolonged into a 

 salient beak, on the mesial line of the back. Septa formed of lobes gene- 

 rally divided into unequal parts, saddles equal. Dorsal lobe longer than 

 the superior lateral one. Lateral saddle less elevated than the others. 

 Dorsal saddle very high. This group differs from the Arietes by the 

 ribs bifiircating, or being ornamented about the middle with points, in- 

 stead of being simple, by the lateral saddle being lower than the dorsal, 

 and not more elevated than the latter, and by its very long dorsal saddle. 

 It differs from the Falciferi in its elevated ribs, ornamented with points 

 and without angles, by the dorsal lobe being very long, instead of short, 

 and by the dorsal saddle, which is of immoderate length. This group 

 is composed entirely of species peculiar to the Cretaceous formation (A. 

 Ilelius, Ixion, cultratus, of the lower stage of the Neocomien ; A. Roissy- 

 iius, cristatus. Bouchardianus, Delarnei, inflafuSf varicosus, Senequieri, 

 Hugardianus, of the Gault, and Lower green sand ; A. Bravaisianus, iri- 

 carinatuSf and varians,* of the chloritic Chalk, or Upper green sand.) 



SPECIES HAVING THE BACK CANICULATED. 



4th Group. TuBERCLTLATi, d'Orb. — ornamented laterally, with 

 ribs and tubercles, these alternating on the sides of the back. Back fur- 

 nished, on the mesial line, with a deep and distinct canal. Mouth com- 

 plete presenting an elongated beak, corresponding to the dorsal canal. 

 Septa formed of lobes and saddles divided into unequal parts. Dorsal 

 lobe shorter than the superior lateral lobe, and so narrow, that it only 

 occupies the width of the dorsal canal. All the species of this group 

 which is very limited, belong to the middle Cretaceous rocks {Am. tuber- 

 culatus, lautus, auritus, of the Gault, or Lower green sand, A.falcatus of 

 the Chloritic chalk). 



SPECIES HAVING A SHARP BACK, BUT NOT KEELED. 



5th Group. The Clypeiformi, d^Orb.~/S^M/ compressed, 

 generally smooth, or slightly wrinkled. Back sharp, without 

 a keel. Spire having the whorls large, generally embracing. 



* This last species has the dorsal lobe very short, without having any 

 of the other characters of the Falciferi ; it may perhaps belong to a small 

 group peculiar to the U})per green sand. 



