574 



MOLLUSCA 



SUB-KINGDOM vi 



FlG 1194 



Ptilocera* planorUs, Sowb. gp. 



the sutures acquire phylliform marginals like those of Phyllocampyli ; but this is not in- 

 variably the case, and characters of the dorsal and 

 lateral sutures as well as of the shell vary in different 

 directions. In one direction they join with Caloceras 

 at the base of the present sub-order ; in another they 

 unite with Angulatida, as recognised by most observers ; 

 and in a third they show affinities with Agassiceras. 



Psiloceras, Hyatt (Fig. 1194); Parapsiloceras, 

 gen. nov. Type P. (Psil) calliphyllum, var. poly- 

 cyclum, Waliner. Lower Lias. 



II. ANGULATIDA. 



Primitive discoidal shells indicate probable union 

 with Psiloceras, but tend to evolve forms in which the 

 venter is smooth, channeled, or crossed by costae at 

 some stage of growth even when keeled. Lateral 

 sutures more complex than in Arietida, and apt to be 

 inclined apicad in specialised shells; dorsal sutures 

 infra- ^^ e those of Arietida. 



Family 2. Angulatidae. More or less com- 

 pressed and costated shells, the costae sometimes 



crossing the venter in the young or extreme age, but usually interrupted in the adult 



by a smooth and occasionally sunken median zone. Sutures inclined apicad near 



lines of involution, more complex in outline 



than in typical Arietidae, and with phylliform 



marginals more like those of Psiloceras. 



Ventral lobe broader and shorter, with larger 



siphonal saddles than Arietidae, and anti- 



siphonal lobe bifid, longer, and more complex. 



First pair of dorsal saddles large and long, 



other dorsal inflections variable, but generally 



more numerous than in Arietidae. 



Schlotheimia, Bay le (Fig. 1195); Waehnero- 



ceras, Hyatt. Lower Lias. 



Family 3. Polymorphidae. Haug. 



Shells compressed discoidal, with smooth young 



like those of Psiloceras. Costae apt to be in- 

 clined or slightly sigmoidal, and continuous 



across the venter. This is crenulated in primi- 

 tive forms, but becomes smooth, channeled, or 



keeled in specialised shells. The latter have 



sutures similar to those of Angulatidae and Liparoceratidae, but less complex. 



with Psiloceras through Gemmelaroceras. 



Polymorphites, Sutner ; Amphiceras, Gemm. ; Uptonia, Tmetoceras, Buckm. ; 



Gemmellaroceras, gen. nov. Type G. (Aegoc.) aenigmatum, Gemm. sp. Lias and 



Inferior Oolite. 





L werLias; 



Connects 



III. ARIETIDA. 



Primitive discoidal, forms keeled and often channeled, with sides costated as in Coro- 

 niceras, etc. External sutures limited, as a rule, and auxiliaries rise orad toward lines 

 of involution, but may be considerably extended, and number of inflections increased in 

 highly involute shells. Dorsal sutures still more limited, and never have the monophyllic 



