446 A. E. Trueman — The genus Polymorphites. 



specimens. In a number of cases slight modifications occur; for 

 example, in some forms striae are developed before the low folds 

 (as in Fig. 8). In one example, the outer whorl of which was 

 similar to P. Jupiter, the inner whorls to a diameter of 9 mm. had the 

 characters of P. caprarius. This suggests that the forms known here 

 as P. cf. jupiter may be polyphyletic, being catagenetic descendants 

 of several more highly ornamented forms. 



The sutural development (shown in Figs, \0a-k) does not vary 

 greatly in the different species considered. The first suture shows 

 the narrow external or azygous saddle flanked by small lateral saddles 

 (Fig. 10a). In the second suture this external saddle is divided by 

 a low ventral lobe, a primitive stage which is omitted in the develop- 

 ment of more highly specialized ammonites. In the third suture the 

 ventral lobe is much deeper and is already divided by a small median 

 saddle ; the external and lateral saddles are by this time higher, 

 though still not so high as wide (Fig. 9c). Not until the diameter 

 of 1 mm. is attained is there any marked advance on this stage, 

 a small auxiliary saddle being then developed (Fig. lOd). At the 

 diameter of 4*5 mm. another auxiliary is present and the first 

 indication of frilling has appeared on the dorsal side of the external 

 saddle. By the fifth whorl (at a diameter of 8 mm.) the external 

 saddles show the well-marked divisions which are characteristic of 

 the adult. It will be noticed that the lateral elements appear to 

 have swung forwards, so that the anterior borders of the external and 

 lateral saddles are approximately level, although the external saddle 

 is much larger than the first lateral. 



Connexions with other Genera. 



Several views have been expressed concerning the systematic 

 position of the species considered. For instance, Blake referred 

 A. trivialis to Amaltheus, the knotted keel of the young possibly 

 being his main reason, while "Wright referred them to the compre- 

 hensive genus JEgoceras. 



Important progress in studying the relationships of Polymorphites 

 and allied forms was made by Haug, who showed that the "keeled" 

 young was quite unlike a young JEgoceras, and suggested that 

 Polymorphites was descended from Agassiceras miser ahile. The simple 

 but wrinkled suture shown by this ammonite, however, can scarcely 

 be compared with the early suture of Polymorphites. Haug further 

 suggested that the evidence of genetic connexion is confirmed by the 

 occurrence of asymmetric sutures in each of these genera, but this 

 character is rarely sufficiently constant to be used as a test of affinity, 

 and, moreover, asymmetry is not common in British specimens of 

 Polymorphites. 



More recently Buckman has suggested that Cymbites is the radical 

 of Polymorphites, 1 and the facts of development support this view. 

 Thus, the shell form of P. Jupiter at a diameter of 1 mm. closely 

 resembles that of Cymlites globosus at 10 mm. diameter. A comparison 

 of the sutural developments of P. cf. Jupiter and C. globosus (see 



1 S. S. Buckman, " On the genus Cymbites" : Geol. Mag., 1S94, p. 361. 



