ORDER II AMMONOIDEA 579 
umbilical but no true lateral zones. Some genera have the latter condition persistent 
throughout life. Costae apt to be continuous across the venter even in specialised forms 
with both channels and keels. Some genera, however, have costae divided on the venter 
or suppressed, but there is no continuous median channel. Keel solid when present. 
Family 13. Dactylioidae. Discoidal forms with costae bifurcated and always 
crossing the venter. Sutures with very complex outlines, but only three or four pairs 
of lateral lobes and saddles. Dorsal sutures have two pairs of saddles and one pair of 
zygous lobes. 
This series is usually termed the Planulati of the Lias, but although an offshoot of the 
same common stock it is quite distinct from its supposed congeners of the Middle and Upper 
Jura. Sutures are straight, not inclined apicad as in Perisphinctidae. The family com- 
. ie =a . 7 vi . . 
prises a complete cycle of forms varying from the broad trapezoidal, tuberculated volutions 
of Coeloceras through Armatoid species to Dactylioceras, in which the costae are smooth and 
8 I d 
sometimes even single. 
Pimelites, Diaphorites, Fucini; (?) Praesphaeroceras, Levi; (?) Collina, Buckm. 
Middle and Upper Lias. 
Family 14. Stepheoceratidae. Buckman. Primitive radicals, highly coronate, 
discoidal, giving rise apparently to involute and partially compressed forms that in 
Coeloceras, Deroceras (Fig. 1205), Dactylioceras (Fig. 1206), Peronoceras, Hyatt ; 

Fic. 1206. 
Dactylioceras commune, Sowb. sp. Upper Lias ; 


England. 
B 
Fic. 1205. = 
ae Gat 
A, Deroceras subarmatum, Young sp. SA 
Whitby, Yorkshire. B, Suture-line of Sphaeroceras Brongniarti, Sowb. sp. Inferior 
Coeloceras pettos, Quenst. Middle Lias. Oolite ; Bayeux, Calvados. 
Macrocephalites and some others are without tubercles. Venter always rounded, 
costae bifureating on the sides and continuous across the venter. Only one line of 
nodes or tubercles at the umbilical shoulders, and division of costae takes place along 
these lines in most forms. Sutures of the same type as in Dactylioidae, but much 
more complex, with usually more inflections, and lobes and saddles more nearly 
equal. Dorsal sutures generally have three pairs of zygous saddles and two pairs of 
lobes in coronate discoidal forms. 
