252 ^EGOCERAS, ARIETITES. 



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

 Peltoceras, Aspidoceras. 



Genus .EGOCERAS, 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 geological!}' younger 

 forms somewhat 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 ma}' be denned, upon the proper refer- 

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

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

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

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

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

 moreum, Opp., and allies ; a third is formed by the t}~pical 

 Capricorns and Armata, from which Stephanoceras with JEgoceras 

 or Stephanoceras pettos and Davoei have developed ; a fourth, 

 going back to the preceding series is represented by the Falcoida 

 which stand on the border towards Harpoceras. JEg. taylori, 

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

 whorls of which indicate their affinity to jEgoceras. 



The true JEgoceras died out in the middle Lias. 



Four Cretaceous, fifty-four Liassic species. 

 ^EGOCERAS BIFERUM, Quenst. T. 107, figs. 609, r>lo. 



Genus AKIETITES, Waagen. 



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

 straight ribs, those on the external angle often angular or in 

 curves directed forwards, frequently spinose. External side 

 carinate, often witli two furrows at the sides of the keel. 

 Margin of aperture simple, straight at the flanks, produced into 

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

 cli:niil)er einhi'iicino- one to one :ind :i-<|ii:irt er whorls. 



