142 A. E. TRUEMAN 



certain ornament stages in the development of Ammonites. The 

 normal order of appearance of ornamentation in Ammonites, as 

 in many other groups is {a) striae, {b) costae, (c) tubercles. But 

 although this order is followed in phylogeny almost invariably, 

 and often, in a general way, in ontogeny, in certain "accelerated" 

 members of phylogenetic series the striate and occasionally also the 

 costate stages are omitted in development, the tuberculate stage 

 having been accelerated so that it directly succeeds the smooth stage. 

 The smooth embryonic stage of an Ammonite may be restricted to 

 the first few whorls, but however accelerated the specimen, the 

 inner whorls appear to be always smooth; the smooth stage is 

 never skipped, while the later costate or striate stages may be.^ 



Similar examples may be found among the Brachiopods and 

 Lamellibranchs. Dr. Lang points out to me that in certain 

 advanced corals twelve septa appear simultaneously in the embryo, 

 the early phylogenetic stages, with fewer septa, being skipped.^ 



THE OMISSION OF COMPARATIVELY LATE CHARACTERS 



A more interesting form of lipopalingenesis is illustrated by 

 certain Ammonite groups; perhaps the clearest example is to be 

 found in the evolution of the Liparoceratidae.^ Slender " Capri- 

 corn " Ammonites of this family evolve through intermediate stages 

 to stout bituberculate forms, but the ontogeny of the latter shows 

 no stage in any way comparable with their immediate ancestors, 

 the slender Capricorn forms. This skipping of the slender-whorled 

 stage is represented diagrammatically in Figure i , which shows 

 sections of three Ammonites typical of {A) the slender Capricorn 

 forms, {B) the intermediate forms, (C) the stout forms, from one 

 genetic series. In the ontogeny of the Capricorn form the depressed 

 early whorls (i and ii) pass into round whorls (iii-v) ; in {B) the stage 

 with depressed whorls is shortened (by acceleration of the slender 

 whorl stage) but the latest whorls are stouter than in the adult of 

 A . This stout form of whorl is more characteristic of C, and has 



^ In certain accelerated Gastropods striae may be present on the protoconch 

 and the smooth stage is there omitted (Grabau, op. cit.). 



2 See Geological Magazine, N.S., Dec. V, Vol. IX (1912), p. 557. 



3 A. E. Trueman, "The Evolution of the Liparoceratidae," Quart. Jour. Geol. Soc, 

 Vol. LXXIV (1919), pp. 2, 7. 



