292 DR. A. E. TEUEilAX OX THE [vol. lxxiv, 



The early evolution of the Echioceratidae and the Liparoceratida? 

 is parallel, both families developing Capricorn ornamentation at 

 approximately the same time. In subsequent evolution, however, 

 while the Liparoceratidae become expanded and bituberculate, the 

 Echioceratidae always remain slender, never develop bituberculation, 

 and in later stages acquire a carina. Although the early evolution 

 of the two families was parallel, separation must have occurred at a 

 comparatively early date, for the sutures, even in the young forms, 

 are very distinct. 



It is extremely probable that the Hildoceratida? arose from a 

 similar stock at a later date ; the early evolution resembles that of 

 the Echioceratidae and the sutures are similar, but the carina 

 becomes more prominent. The Hildoceratidae may comprise several 

 parallel lines of descent from a f/lobosus-like form. Some specimens 

 of Frechiella, a globose ammonite Avith keeled venter (1, vol. i, 

 No. 23), have typical Hildoceratan sutures and may be intermediate 

 between the ancestral form and one of the later branches of the 

 Hildoceratidse. 



It seems probable, therefore, that the Liparoceratidae, Echiocera- 

 tidae, Hildoceratidae, Polymorphidae, 1 and Deroceratidae 2 ma}' have 

 developed directly from a globose ancestor. Other families, how- 

 ever, while the} 7 have probably descended from a similar globose 

 form, have passed through a subsequent cadicone stage ; thus Hyatt 3 

 regarded Coeloceras pettos, a broad-ventered (cadicone) tuberculate 

 ammonite, as the acmic tertiary radical of the Dactyloidae. Some 

 members of the Dactyloidae pass through a peftos-like stage with 

 cadicone whorls and smooth venter. 4 Later Dactyliocerates omit 

 the cadicone stage altogether in development, an omission which is 

 comparable with the skipping of the Capricorn stage in the develop- 

 ment of the later Liparoceratidae. 



The JBeaniceras series, to which reference has already been made, 

 passes from a tuberculate cadicone to a Capricorn, similar in outward 

 appearance to the anagenetic capricorns of the Liparoceratidae. 

 Mr. S. S. Buckman, who placed Beaniceras with the Liparoceratidae 

 (1, vol. ii, p. in), now agrees with me that its affinities are with 

 the Dactyloids, which it resembles in sutural characters and in the 

 cadicone ancestry. 



The Amaltheidae likewise pass through a cadicone stage, and 

 may perhaps be considered as related to the Dactyloids rather than, 



1 A. E. Trueman, 'Observations on the Genus Polymorphites ' Geol. Mag-, 

 dec. 6, vol. iv (1917) p. 442. 



2 The Deroceratidaa have sutural characters (particularly a deep suspensive 

 auxiliary lobe) which separate them from the greater part of the Lias 

 Ammonites. 



3 A. Hyatt, ' The Genesis of the Arietidre ' Smithsonian Contributions to 

 Knowledge, No. 673 (1889) p. 23. See also A. Hyatt, ' The Non-Reversionary 

 Series of the Liparoceratidse, &c.' Proc. Bost. Soc. Nat. Hist. vol. xv (1S72-73) 

 p. 4. 



4 Coeloceras fontieulum has a pettos-\ike stage from the diameter of 5 mm. 

 to that of 10 mm. After this stage ribs appear on the venter. Some species 

 of Porpoceras and Perotioceras show corresponding stages. 



