194 



THE LIAS AMMONITES. 



But the Ammonites of the St. Cassian (Triassic) beds with Amvionites Aon, Mtinster, 

 were not comprehended in a distinct group. 



M. Alcide d'Orbigny divided the Ammonites into twenty-one sections -^ in these he 

 inckided the groups established by von Buch, and added others to receive the many new 

 forms which he had discovered among the very rich materials he had assembled for his 

 great work on the Terrains Cretacee and Terrains Jurassique of France. 



a. Species with a sipTional heel entire, 

 1st Section. Arietes, von Buch. See page 186. 

 2nd Section. Falcifeki, von Buch. See page 187, 



3rd Section. Cristati, d^Orhigny. Shell compressed, sides ornamented with ribs, 

 bifurcated and bent forwards without forming an angle ; with or without prominent tuber- 

 cles. Siphonal area prominent in the keel, which contains the siphuncle. Mouth, when 



Fig. 74. — ScMoenbachia Roissyana, d'Orb. 



Fig. 75. — ScMoenbachia cristata, Deluc. 



perfect, prolonged into a prominent rostrum upon the median line of the margin. Septa 

 formed of lobes, in general divided into unequal parts and into equal saddles. Siphonal 

 lobe longer than the superior lateral. The lateral saddle less elevated than the others. The 

 siphonal saddle very high. The best types are ScMoenbachia cristata, Deluc (fig. 75), 

 and Schloenhachia Boissyana, d'Orb. (fig- 74), both from the Cretaceous Formations. 



h. Species with the siphonal area canaliculated. 



4th Section. Tuberculati, d' Orbigny. The sides of the shell ornamented with ribs 

 and tubercles. Siphonal area provided in the middle with a deep well-defined channel. 

 Mouth, when complete, representing an elongated rostrum corresponding to the siphonal 

 ^ ' Paldontologie Fran9aise; Terrains cr^taces,' torn, i, p. 405, 1840. 



