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, VOrb. 
(Type H. dissimilis, VOrb.) ; Pietetia, Uhlig ; 
Helicancylus, Gabb; Distoloceras, gen. nov. 
Type D. (Hopl.) hystrix, Neum. and Uhlig 
sp.; Acrioceras, gen. nov. Type A. (Ancyl.) 
Tabarelli, 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 
Fic. 1230, tubercles. 
Ancylocerus Matheronianum, VOrb. Neocomian ; Crioceras, 
seeing, Basses Alpes. A, Conch. B, Suture- Orb. 

(Criocera- 
tites, Lev.) ; Oxy- 
beloceras, gen. nov. Type O. (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 — Bostrychoceras polyplocum, 
in some cases complete, so that aperture is brought close Heiden, Westpiabe, Cretaveouns 
against base of the spire. Young of Himperoceras may 
remain Hamitean for a prolonged period, and then suddenly become Helicoceran or 
turrilitiform. : 
Nostoceras, Emperoceras, Didymoceras, Exiteloceras, Hyatt; Dostrychoceras, gen. 
nov. (Fig. 1231). Type B. (Heteroc.) polyplocum, Schliit. sp. Cretaceous. 

Fic. 1231. 
e 
XJ. MamaMtrripa. 
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. Ditfer from Pedioceratidae in that the venter 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. ; 
