A. E. Trueman — The genus Polymorphites. 443 



in the table. The numbers there given refer to the ornament cycle 

 of Buckman. 1 



Occurring with these species at Old Dalby is a form which 

 outwardly resembles P. jupiter, but which has costate inner whorls 

 and is probably a catagenetic offshoot of P. trivialis. A similar 

 example was figured by Buckman, 2 who considered it to be P. jupiter 

 showing temporary catagenesis, but who now suggests 3 that the 

 persistence of the striate ornamentation indicates that it is a different 

 species. For convenience this degenerate form is here referred to as 

 P. cf. jupiter. 



USr»«.) 



Fig. 11. — P. mixtus. s, last suture 

 m, mouth margin. 



lis m*n.) 



tl*J mm) 



a 



Fig. 12. — Adult sutures of Polymorphites. 

 a, P. trivialis ; b-d, P. mixtus. 



The adult sutures of some specimens of P. trivialis and P. cf. jupiter 

 have saddle terminations more rounded than others (Fig. 12«), and 

 the external saddle may have either three or four terminal folioles. 

 In nearly all cases the last three or more sutures are closely crowded 

 (Figs. 1, 11), a feature which is also shown in some of Quenstedt's 

 figures. 



In examples of Polymorphites the mouth margin is not usually 

 preserved, but a specimen of P. mixtus shows a mouth border 

 preceded by a constriction (Fig. 11); in this case the length of the 

 body-chamber is half a whorl. 



1 S. S. Buckman, Yorkshire Type Ammonites, vol. i, p. xiii. 



2 Ibid., No. 53c. 



3 Inlitt., Augusts, 1917. 



