A. E. Trueman — The genus Polymorphites. 445 



J. F. Blake 1 pointed out that in the Yorkshire examples of 

 A. trivialis the inner whorls are not usually preserved. The earlier 

 chambers, however, are present in occasional specimens from Old 

 Dalby, and the development may be studied without much difficulty 

 in P. cf. jupiter. 



The Development of Polymorphites cf. jupiter. (Figs. 1-9.) 



The protoconch, which is not often preserved, has axes of *56 mm. 

 and *4mm. respectively, and shows the usual angusti-sellate characters. 

 The whorl section is depressed in the early stages (see Figs. 4 and 5), 

 and does not become as high as wide until a diameter of 4 mm. is 

 reached. Before this time (at about 1 mm. diameter) a somewhat 

 angular (convexi-fastigate) venter is developed (Fig. 6), and at 

 a diameter of 2 mm. low curved folds appear. During the next 

 whorl the venter becomes more angular and the folds more prominent, 

 especially on the venter, where they bend sharply forwards (Figs. la, b), 

 causing a superficial resemblance to the crenulate keel of the 

 Amaltheidse. 



( 8 hm.) 



a 



C. (Third) 

 Ja. (Second) 



a:(TW.) 



Fig. 10. — Sutural development, P. cf. jupiter. 



This costate stage persists until the shell attains a diameter of 

 6-9 mm., when it is succeeded by a subcostate stage, low folds 

 alternating with striae of similar form (Fig. 8), which presently are 

 developed to the exclusion of the folds (Fig. 1); by this time whorl 

 height appreciably exceeds whorl thickness and the venter is rounded 

 or slightly angular. In later life the striae become less prominent. 



The account of the development just given applies to most of our 



1 In Tate & Blake, The Yorkshire Lias, 1876, p. 292. 



