252 jEGOCERAS, ARIETITES. 



phites, Hoplites, Acanthoceras, Stoliczkaia, Crioceras, Heteroceras, 

 Peltoceras, Aspidoceras. 



Genus .ffiGOCERAS, Waagen. 



Shell mostly compressed, composed of many whorls, embra- 

 cing but little, sometimes provided with nodose or externally 

 bifurcate ribs ; never with true sickle-like ribs ; not carinate ; 

 body-chamber usually a whorl long, in the geologically younger 

 forms somew r hat shorter. Aperture simple without lateral ap- 

 , pendages, with very weak external lobes and a constriction ; a 

 single corneous aptychus. Lobular line strongly notched, upper 

 lateral longer than the siphonal, lower lateral not always present; 

 usually with a depending siphonal lobe. Lobular bodies narrow. 

 not wedge-shaped ; anti siphonal two-pointed. 



Several series of forms may be defined, upon the proper refer- 

 ence of which further investigations remain to be made; one of 

 these is that of JEg. incultum, Beyr., to which JE<j. palmai, Mojs., 

 Buonarottii, Mojs., planorbis, Sow., Jolmstoni, Sow., planor- 

 boides, Sow., etc., are related; a second series belongs to yfy/. 

 svbangulare, Oppel., angulatum, Schl., Gharmassei, Orb., mar- 

 moreum, Opp., and allies; a third IK formed by Hie typical 

 ( 'a pricorns and Armata, from which Slephanoceras with y/t/V/orr/v/x 

 or Stephanoceras pettos and Davoei have developed ; a fourth. 

 going back to the preceding series is rep resented by the Falcoida 

 which stand on the border towards Harpoceras. jEg. ItnjJori. 

 Henleyi, altermim, Opp., are somewhat aberrant forms, the inner 

 whorls of which indicate their allinity to JEgoceru.x. 



r riie true JEgoceras died out in the middle Lias. 



Four Cretaceous, fifty-four Liassic species. 

 ^EGOCERAS BIPERUM, Quenst. T. 107, figs. Co ( .>. r, in. 



<l'iius ARIETITES, Waagen. 



Shell flat, discoidal, with wide nmbilicns; on the flanks simple 



Straight ribs, those on the external angle often an<rnlar or in 

 curves directed forwards, frequently spinose. Kxternal side 

 carinate, ofli-n witli two fin-rows at tlie sides of the keel. 

 M Mi-gin of ajtertui-e simple, st might, at the Hanks, produced into 

 a pretty long, pointed lobe, which is never bent inwards; body- 

 chamber embraeing one to one and a-<|iiartcr whorls. 



