﻿Vol. 6 1.] GENERA AND SrECIES OE LYTOCERATID^S. 143 



cycle to which the Lytoceratacea belong. The whole Cephalopod- 

 cycle is from straight to coiled up (involute) and back to straight 

 again; but within this main cycle are found many subsidiary 

 cycles, of successive retreat and advance, from involute to evolute, 

 on again to involute, and so on. Through the phases of one of 

 such subsidiary cycles the Lytoceratacea pass. Like the Am- 

 monacea, they have evidently come through an involute, Goniatitic, 

 stage ; they revert to the evolute stage, and advance again towards 

 involution — the evolute is therefore more primitive than the later 

 involute. However, in reverting towards evolution and the 

 Orthoceratan style, the Lytoceratidse went much farther than the 

 Phylloceratidae, and they retain the evolute character longer. 

 Then, changing this process, going again towards involution, the 

 Phylloceratidae begin first, at a time when their lobe-line is still 

 more primitive than that of the Lytoceratidse . The latter, starting 

 later on the same way, seldom progress so far towards involution 

 as the former ; but the interesting species which I have to describe 

 is the most noticeable case of how far they do travel in that 

 direction. 



Now, in the jurense-groivps — to one of which this species belongs 

 — the development in regard to ornament presents, more or less 

 completely, these stages: — subcostate, costate (crassicostate, or 

 corrugate), returning to subcostate, and levigate. But the develop- 

 ment of the phyletic series does not end here. The levigate 

 character is attained while the species are somewhat evolute ; it is 

 retained while they become (in one case) truly involute (Phyllo- 

 ceratan). These successive stages of development may be indicated 

 in terms of species : — (1) Germaini, (2) torulosum, (3) annulose, 

 the species not yet named, 1 (4)jurense 9 (5) phylloceratoidan ; and 

 the species mentioned are adults, more or less in these respective 

 stages of development. 



That the species like jurense were levigate developments of 

 Germaini-like forms I have pointed out before. 2 But a cursory 

 examination of these species shows that they belong to more than 

 one genetic series, that the smooth forms are polygenetic, and have 

 passed through independent costate stages. Two of such genetic 

 series Hyatt has separated as genera : Pleurolytoceras, the hircinum- 

 group ; Alocolytoceras, the Germaini-grouip. 3 To neither of these can 

 the torulosum -group be fitted. Although torulosum shows that it 

 came through a Germaini-like stage, yet its corrugate stage differs 

 from that of the Germaini series, and its levigate developments 

 differ from those of that series also. It seems, therefore, necessary 



1 Lytoceras Germaini, Janensch {non d'Orbigny), Abb. z. Geol. Specialk. 

 von Elsass-Lothringen, n. s. pt. v (1902) pi. ii, fig. 3 only, shows this stage. 

 Compare also Lytoceras diliocidum (Oppel), Pompeckj. 



2 ' The Reported Occurrence of Ammonites jurensis in the Northampton 

 Sands' Geol. Mag. 1892, p. 260. Dr. J. P. Pompeckj also has given much in- 

 formation concerning the development of these forms, in his ' Beitrage zu einer 

 Revision der Ammoniten des Schwabischen Jura' pt. ii (1896) pp. 95 et seqq. 



3 Eastnian-Zittel, ■ Text-book of Paleontology ' vol. i (1900) p. 572. 



