568 



MOLLUSCA 



SUB-KINGDOM VI 



D. (Phyl.) rakosense, Herbich sp. ; Schistophylloceras, gen. nov. Type S. (Phyl.) 

 aulonotum, Herbich sp. ; Geyeroceras, gen. nov. Type G. (Phyll.) cylindricum, Geyer 

 sp. ; Tragophylloceras, gen. nov. Type T. (Phyl.) heterophyllus-numismalis, Quenst. 

 sp. ; Meneghiniceras, gen. nov. Type M. (Phyl.) lariense, Menegli. sp. Trias to 

 Cretaceous. 



Sub-Order H. LEPTOCAMPYLI. Hyatt. 



Shells with peculiar, more or less crenulated ornamentation, due to the intersection of 

 two systems of lines, transverse bands of growth, and longitudinal creases and ridges, 

 sometimes developing into spines. Aspect more discoidal and larval, and with fewer 

 highly involute forms than in other sub- orders. Shell often constricted as in Phyllo- 

 campyli. Characterised in some families by the small number of completely developed 

 lobes and saddles. Outlines of sutures extremely complex, and saddles generally much 

 reduced owing to excessive development of the lobes. 



The antisiphonal lobe has in most families a cruciform aspect, due to the develop- 

 ment of a large pair of branching marginal lobes. In the Lytoceratidae there is also 

 an inward growth of the two marginals near the tips, so that they rest upon the oral 

 surfaces of the septa instead of against the inner surface of the dorsum ; these modi- 

 fications being peculiar to this sub - order. The principal inflections are more 

 numerous in the young, and reduction takes place in later stages through arrested 

 development and suppression of those nearest the lines of involution. Wahner's 

 thorough researches have shown the close affinities of Pleuracanthites with Psiloceras 

 and Caloceras, which lie at the ancestral base of this sub-order. This accounts for the 

 similarities between the young of this group and Phylloceratidae, as demonstrated by 

 J. P. Smith. A number of phylogerontic uncoiled genera occur in the Cretaceous. 



Family 1. Pleuracanthitidae. Discoidal, plicated, or costated shells, having 

 crescentic tubercles or hollow spines in some species ; venter rounded except in extreme 

 age of a few species, in which a faint keel appears. Aperture has a short rostrum 



with deep ventro - lateral 

 or lateral sinuses, and 

 lateral or dorso - lateral 

 crests. 



Pleuracanthites, Cana- 

 vari ; Analytoceras, gen. 

 nov. Type A. (Lyt.) 

 articulatum, Wahner sp. 

 Lias. 



Family 2. Tetra- 

 gonitidae. Smooth, 

 discoidal, and involute 

 shells, with flattened or 

 rounded venter, and more 

 or less flattened sides. 

 Constrictions and spines 

 sometimes present. Sutures 

 sometimes reduced to four 

 paired saddles and three 

 lobes on the dorsum, -or 

 there may be only two 

 pairs, or a single pair of 

 large saddles. Siphonal lobes deeper and broader than in Lytoceratidae, the siphoiial 

 saddles larger, and laterals more numerous, comprising sometimes as many as nine pairs oi 

 complex zygous lobes and saddles. Antisiphonal lobe straight, complex, trifid, or bifid. 



Fio. 1175. 

 Haploceras elimatum, Oppel sp. Tithonian ; Stramberg, Moravia. 



