part -4] EYOLUTION OF THE LIPABOCEBATLD.X. 291 



The former view has been maintained by Air. Buck man, 1 who 

 suggested that Cymbites is the ancestor of many Mesozoic ammo- 

 nites ; the inner whorls of ammonites are remarkably similar 

 in form to those of the adult Cymbites, hut also the sutural 

 development of many ammonites suggests that they arose from a 

 Cymbites-Wke ancestor. It must further he noticed that there are 

 numerous transitional forms which appear to connect globose 

 ( Glyphioceras-like) species with ammonites of many families. 2 a 

 fact which appears to indicate the truth of Mr. Buckman's 

 suggestion, but cannot easily be reconciled with the second view 

 put forward by Prof. J. P. Smith/' 



These globose ammonites have generally been called Cymbites 

 glohosus : the name has probably been used, however, to cover several 

 species which differ little in outward form, but have a long vertical 

 range 4 ; from this central group the various ammonite-families 

 may have arisen at different times. 



The Triassie ancestor of Cymbilc* (and so, perhaps, of most 

 Jurassic ammonites) is probably an involute form, with a depressed 

 whorl and simple suture comprised of few elements. Mr. Buckman 

 suggested that Nannites may be the ancestral form"' ; but, as this 

 acquires constrictions in the adult, 6 which are not reproduced in 

 Cymbites, it is probably only safe to assume that it is near the line 

 of descent of the ammonites. 



General Considerations. 



A genealogy of Jurassic ammonites was given by Mr. Buckman 

 in 1898. 7 Since that date his own views have undergone con- 

 siderable modification, and it must be stated that some of the 

 differences between that genealogy and the one given here have 

 been suggested to me by Mr. Buckman's subsequent writings. 



1 ' On the Genus Cymbites ' Geol. Mag. dec. 4, vol. i (1894) p. 360. 



2 For example, in the nodules of the planicosta sub-zone of Dorset the 

 following series may be found : 



(1) Cymbites sp. of the C. -laevigatas pattern, more evolute than C. globosvs. 



(2) Cynibites sp. of the C. berardi pattern, with broad lateral costae and 



smooth periphery. 



(3) Ammonites with broad costae, which pass across the periphery. 



(4) Ammonites of the planicosta group, frequently smooth up to a diameter 



of 8 mm. 



3 If the 'skipping of stages' demonstrated in this paper has been of 

 frequent occurrence, it may often prove impossible to judge ancestry from 

 ontogenetic evidence, and the tracing of numerous connecting forms will be 

 necessary in determining genealogi 



4 Thus the sutural development of examples from the Middle Lias proceeds 

 more rapidly than that of specimens from the Lower i 



5 Op. supra rit. 1894, p. 360. 



fi A. Hvatt & J. P. Smith. ' Triassie Cephalopod Genera of America' L'.S. 

 Geol. Surv. Prof. Paper, No.' 40 (1905) p. 78. 



7 S. S. Buckman, ' Divisions of so-called Jurassic Time ' Q. J. G. S. vol. liv 

 (1898) table ii, facing p. 4ol. 



