﻿Vol. 6 1.] 0ENEKA AND SPECIES OF LYTOCEEATIDiE. 151 



Other species are : — 



Megalytoceras rubescens (Dumortier) = Am. rubescens, Dumortier, 



'Depots Jur. du Bassin du Rhone ' pt. iv (1874) p. 114 & 



pi. xxix, fig. 5 ; & Vacek, * Die Fauna der Oolithe von 



Cap 8. Yigilio ' Abhandl. d. k.-k. Geol. Eeichsanst. vol. xii 



(1886) p. 63 & pi. i, fig. 5. 

 Megalytoceras sp. = Lytoceras, n. sp. indet., Vacek, op. cit. 



p. 64 & pi. i, figs. 6-7. 

 ? Megalytoceras rasile (Vacek) = Lytoceras rasile, Vacek, op. cit. 



p. 63 & pi. iii, figs. 5-8. 

 Megalytoceras amplum (Oppel) = Ammonites arnplus, Oppel, 



< Pal. Mitth.' pt, iii (1862) p. 145 & pi. xlv. 



Genus Nannolytoceeas, 1 nov. 

 Genotype, Ammonites pygmmis, d'Orbigny. 2 



Definition. — A primitive Lytoceratoid genus; whorls smooth, 

 e volute, compressed, with distant, ill-marked periodic constrictions. 



Distinction. — From Lytoceras and like genera by the absence 

 of sculpturing and ' flares ' ; from Pachylytoceras and like genera 

 by the association of smoothness with an evolute, compressed whorl. 

 In those genera the smoothness is a catagenetic feature ; in this 

 genus all the evidence points to its being an anagenetic character — 

 the genetic series not having passed through the ornate stages. 



Comparison of Superior Lateral Lobes. 



The accompanying figures (p. 152) show the contrasts and affinities 

 of various superior lateral lobes (L), in skeleton outline, omitting 

 minor details. Fig. 2 shows a supposititious normal tniooulate 

 lobe, with the terminal lobule, A, isosceles. Fig. 3 shows the L 

 (outline) of Al. Germaini (Pompeckji), after d'Orbigny ; and if A be 

 rightly interpreted as the terminal lobule, then this lobule is 

 ultra-brachysceles (short-legged on the outer part). The same 

 feature, even more pronounced, is seen in figs. 4 & 5 ; and the 

 greater development of the inner lobule (C) in proportion to the 

 outer (B) causes the lobe to assume a bifid pattern. In all the 

 figs. 3-6 belonging to the^Wewsd-groups, the wide-stemmed character 

 of the lobe and its want of ' spread ' are noticeable — they are all of 

 a similar plan, in marked contrast to the narrow-stemmed, wide- 

 spread lobes depicted in figs. 7-9. In these there is the ultra- 

 brachysceles character of A, but in figs. 8 & 9 it may be noted that B 

 is rapidly enlarging and becoming more equal to C, — in. fact B 

 begins to usurp the functions of the siphonal lobe, see fig. 1 (p. 150). 

 The difference between fig. 9 and figs. 4-6 shows how distinct is 

 Megalytoceras from the jurense- groups, in spite of a somewhat similar 

 aspect. But, if the inferior lateral lobe of Megalytoceras (fig. 1) 

 be compared with the L of the jWense-groups (figs. 4-6), it will be 



1 Navy os, a dwarf. 



2 ' Pal. Franc;. Terr. Jur. : Ceph.' 1846, p. 391 & pi. cxxix, figs. 12-13. 



