252 ^GOCERAS, AEIETITES. 



phiies, Hoplites, Acanthoceras, Stoliczkaia, Crioceras, Heteroceras, 

 Peltoceras^ Aspidocer'as. 



Genus ^GOCEKAS, Waagen. 



Shell mostl}- 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 geologicall3^ younger 

 forms somewhat shorter. Aperture simple without lateral ap- 

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

 single corneous apt^^chus. Lobular line strongl}' notched, upper 

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

 usually with a depending siphonal lobe. Lobular bodies narrow, 

 not wedge-shaped ; antisiphonal 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 ^g. incultum, Bej^r., to which j^g. palmai, Mojs., 

 BuonaroUii, Mojs., planorbis, Sow., Johnstoni^ Sow., planor- 

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

 subangulare, Oppel., anguLatum^ Schl., Gharmassei, Orb., mar- 

 mioreum^ Opp., and allies ; a third is formed by the typical 

 Capricorns and Armata, from which Stejjhanoceras 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 Harpooeras. JEg. taylori^ 

 Henlei/t, aUermcni, 0pp., are somewhat aberrant forms, the inner 

 wliorls of which indicate their affinity to ^gocet-a.-t. 



The true jEgoceras died out in the middle Lias. 



Four Cretaceous, fift3'-four Liassic species. 

 ^'EOOCERAS BIFERUM, Qiicust. T. lOY, figs. 609, 010. 



Genus AEIETITES, Waagen. 



Shell flat, discoidal, with wide uml)ilicus ; on the flanks simple 

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

 curves directed forwards, frequently spinose. External side 

 carinate, often with 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- 

 chamber embracing one to one and a-quarter whorls. 



