256 DR. A. E. TRUEMAN ON THE [vol. lxxiv,. 



generally increasing with more advanced forms ; this constitutes a 

 ready method of determining approximately the stage of advance 

 of any specimen. 



It must be recognized that most Liparoceratids with unpaired 

 tubercles have paired tubercles in their early stages ; but, with 

 acceleration of development, the stage of unpaired tubercles is. 

 reached earlier in successive members of a series (see Table II, 

 D, E), and in very advanced forms the earliest ornamentation 

 acquired may consist of unpaired tubercles, the outer row having 

 double the number of the inner. The beginning of catagenesis, 

 on the other hand, may be shown by a decrease in the ratio of 

 the number of outer to inner tubercles on the outer whorl. 



The following ornament stages may, therefore, be recognized in 

 most lineages : — 



(a) Capricorn ribs, regularly increasing in number on successive whorls. 



(b) Capricorn ribs, widely spaced on the outer whorl. 



(c) Capricorn ribs widely spaced, bituberculation commencing ; prominent 



striations between the ribs on the periphery. 



(d) Ribs more slender, bituberculate, tubercles paired ; ribs dividing on 



the periphery. 



(e) Similar, but with tubercles unpaired. 



(/) Tubercles returning to the paired condition. 



The stages a to e are anagenetic ; stage f is catagenetic. In 

 the later stages of many sphserocones also the tubercles are finer 

 than in the early stage, leading to great similarity in the several 

 series (homceomorphy). Catagenetic ornament, however, is occa- 

 sionally seen in the old age of various capricorns, which do not 

 attain the tuberculate stage, but pass directly from anagenetic 

 costate to catagenetic subcostate and striate stages. 



Successive members of the series are likewise often characterized 

 by increasing involution and by greater complexity of suture-line. 

 Thus the simple sutures of a Capricorn in any lineage are closely 

 comparable with the early sutures of the forms which evolved from 

 it, and with the septal sections of their adult septa. 1 



Longitudinal striations appear to be developed in each of the 

 series considered here, usually appearing first on the periphery soon 

 after the whorls are swollen and bituberculate. In the young of 

 advanced species, such as Liparoceras clieltiense, longitudinal stria- 

 tions are developed at a diameter of 20 mm. The shell is rarely 

 well-preserved, and thin sections do not show details of structure : 

 the slight corrugation of the outer shell-layer which gives rise to 

 the striate appearance is often due to the presence underneath it 

 of sharply-defined parallel ridges varying in width, but rarely more 

 than -5 mm. across, and not more than 1 mm. apart (see -1, pi. xxix, 

 fig. 7 cc). It is possible, however, that in some cases the underlying 

 ridges are not present, or are lost in subsequent development. 



1 H. H. Swinnerton & A. E. Trueman, ' The Morphology & Development of 

 the Ammonite Septum ' Q. J. G. S. vol. lxxiii (1917-18) p. 26. 



