MODERN CLASSIFICATION. 241 



shorter than the principal lateral lobe, which is mostly broad and wedge-shaped. A 

 divided horny Anaptychus has been found in this genus. 



The genus Avialtheiis has an extensive geological range; it is found in the Trias, 

 Amal. meffalodiscus, Beyr., Amal. Jloridiis, Wulf. ; it is largely developed in the Jurassic 

 strata, commencing with Amal. Guibalianus,A'Oth., Amal. margaritatus, Mont. (fig. 134), 

 and Amal. spinatus, Brug., in the Lias, and many forms are found in the higher Oolitic beds, 

 as Amal. cordatus, Sow., Coral Rag (fig. 135) ; numerous species are likewise discovered 

 in the Cretaceous rocks, as Amal. Gevrilianus, d'Orb., Amal. Vibrayeanus, d'Orb. 



Genus Schloenbachia, Neum. — Shell keeled, with strong forward-bent ribs on the 

 sides, sometimes developing lateral tubercles. Body-chamber two thirds of a whorl 

 long. Mouth-border falciform, ventral side with a forward directed process. Siphon 



Fig. M&.SclUuriiijucnia Kuissyana, Fig. 137. — Schloenbachia Fig. IZi-Schloetihachia Goupilanus, 



d'Orb. crisiata. Deluc. d'Orh. 



large, mostly lodged in the high keel (fig. 137); this genus is entirely limited to the 

 Cretaceous rocks; figs. 136, 137, and 138 represent typical forms of the group. 



II. Family. — Tropitid^, Mojs. 



Shell more or less richly ornamented, and provided with radial ribs, which almost 

 always support tubercles or spiny processes on the border of the siphonal area. Wrinkle- 

 layer and impression on the surface of the mantle entirely wanting. 



Genus Tropites, v. Mojs. — Body-chamber from one and a half to one and three 

 quarters of a whorl in length. The strong-ribbed sculpture is interrupted upon the 

 ventral surface, and rises as a median keel on the same. The ventral side of the mouth- 

 border projects forward in a short broad lappet ; the last whorl seldom loses the form 

 and sculpture of the inner whorls. The lobes are characterised by a broad saddle stem 

 with transverse incised branches, a transverse placing of the lobe points, great develop- 

 ment of the chief lobes, and a remarkable diminution of the auxiliaries. All the species 

 are Triassic. Ex. Tropites Pamphagus, Dittm., Trop. Saturnus, Dittm. 



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