262 DR. A. E. TEUEMAN OX THE [vol. lxxiv,. 



Amblycoceras Hyatt, 1900. 



Genoholotype : A. capricornus Hyatt. 1 



Develops from Capricorn to a form with bituberculate swollen outer 



whorl. The costae are very little diminished on the periphery of the 



capricorns, and shw a distinct forward bend, less sharp than that of 



Oistoceras. 

 Suture : IL much shallower than EL, which usually has a short and 



wide median saddle. IL has three subequal terminal lobules. ES 



wide, with three terminal cells (fig". 1/, p. 258). 

 Species included : — 



A. crescens Hyatt, A. dissotypum, sp. nov., A. brevilohatum, sp. nov. 

 Horizon : Davcei zone {latsecosta sub-zone to Oistoceras sub-zone). 



The ammonites of this genus are of two series : the earlier, 

 which includes A. brevilohatum, has slender elevated whorls with 

 close and uniform costa? ; and the later, including A. crescens, has 

 stout whorls, with prominent costse separated by wide concave 

 spaces on the outer whorl. Bituberculate forms are known in 

 both series. The A. crescens series is not derived from the earlier, 

 for A. crescens retains the primitive depressed whorl ; its inner 

 whorls, however, have numerous ribs and are similar in form to 

 the young of A. brevilohatum. It is probable that this latter 

 group represents an offshoot from a primitive crescens-\ike stock. 



Oistoceras S. S. Buckman, 1910. 



Genoholotype : 0. figulinum (Simpson). (Refigured, 1, vol. i, No. 26.) 

 Develops from a Capricorn to a bituberculate form with swollen whorls ; 



the costae are little diminished on the periphery, where they have a 



very marked forward curve, much greater than in Amblycoceras 



(' arrow-like peripheral costae ').' 2 

 Suture : EL and IL about equal in depth, EL narrow and with a high 



median saddle in the later forms ; IL frequently asymmetrical 



(fig. 1 e, p. 258). 

 Species included : — 



0. figulinum (Simpson), 0.alloeotypum,^.noY., 0. omissum (Simpson), 

 0. curvicornum (Schloenbach). 

 Horizon: ? dsedalicosta sub-zone to lower part of margaritatus zone 



(at Lincoln only). Dominant in the Oistoceras sub-zone. 



Two series may be recognized among the ammonites of this 

 genus, namely : — 



(1) With squared venter, developing coarse bituberculation 

 (O. curvicornum Wright, non Schloenbach). N 



(2) With a rounded or subangular venter, developing fine 

 bituberculation (0. allceotypum, PI. XXV, fig. 1). Some forms of 

 the latter series have a peripheral rib-curve not much stronger than 

 that of Amblycoceras ; they may be distinguished from the latter 

 genus, however, by sutural characters. Similarly, occasional speci- 

 mens of Amblycoceras have a rib-curve equal to that of Oistoceras 

 (see p. 279). 



1 In Eastman-Zittel, ' Text-book of Palaeontology ' 1st ed. vol. i (1900) 

 fig. 1204, p. 578. 



2 See 1, vol. i, p. iv. 



