588 



MOLLUSCA 



SUB-KINGDOM VI 



Ancyloceras, Crioceras, etc., as have three rows of tubercles on either side on the larger 



costae. Between the latter are smaller costa- 



tions without tubercles. Costae sometimes 



interrupted across the venter by a smooth 



zone. 



Ancyloceras (Fig. 1230), Hamulina, d'Orb. 



(Type H. dissimilis, d'Orb.) ; Pictetia, Uhlig ; 



Helicancylus, Gabb ; Distoloceras, gen. nov. 



Type D. (Hopl.} hystrix, Xeum. and Uhlig 



sp. ; Acrioceras, gen. nov. Type A. (Ancyl.} 



Tabar'elli, Astier sp. Cretaceous. 



Family 32. Crioceratidae. Volutions 



more or less 



roughened and 



costated, with 



only two lines of 



tubercles on either 



side of the median 



line of the venter. 



The latter may 



have a smooth 



zone or be crossed 



by the costae, 



which are either 



single or double 



between the 



tubercles. 



C r i o c e r a s, 



d'Orb. (Criocera- 



tites, Lev.) ; Oxy- 



beloceras, gen. nov. Type 0. (Pty.} crassum, Whitf. sp. 

 Cretaceous. 



Family 33. Nostoceratidae. Adult spirals 

 asymmetrical turriliticones, with more or less prominent 

 costae and one row of tubercles on each side of the venter. 

 Extreme gerontic stage often retroversal, the return being 

 in some cases complete, so that aperture is brought close 

 against base of the spire. Young of Emperoceras may 

 remain Hamitean for a prolonged period, and then suddenly become Helicoceran or 

 turrilitiform. 



Nostoceras, Emperoceras, Didymoceras, Exiteloceras, Hyatt ; Bostrychoceras, gen. 

 nov. (Fig. 1231). Type B. (Heteroc.} polyplocum, Schliit. sp. Cretaceous. 



FIG. 1230. 



Ancyloceras Matheronianmn, d'Orh. Neocomian ; 

 Castellanc, Basses Alpes. A, Concli. B, Suture- 

 line. 



1231. 



Bostrychoceru* polyplocum, 



Cretaceous; 



XI. MAMMITIDA. 



Shells closely parallel to Cosmoceratida, but differ in having at some stage a median 

 line of tubercles along the venter. These are fused in more specialised genera into a solid 

 entire keel. 



Family 34. Mammitidae. Differ from Pedioceratidae in that the ventvr is 

 rounded and smooth in the young, after which a median ventran row of tubercles 

 appears, or a keel resulting from their fusion. Sutures have larger and more solid 

 saddles than in Pedioceratidae, and first lateral saddles generally bifid. 



Mammites, Laube and Bruder ; Muniericeras, Gross. ; Barroisiceras, Gross. ; 



