SECOND, OR SCHLOTHEIMIAN BRANCH. 131 



their old proportions. In the full adult condition the characteristics of the 

 sutures differ considerably from those of the typical Arietidge, and approximate 

 to those of Psiloceras. 



The seventh whorl increases in size with great rapidity, the abdomen be- 

 coming narrower, the channel shallower, the pilae more depressed, losing their 

 prominent somewhat abrupt genicular bend, and on the abdomen becoming 

 depressed to a level with the siphonal line. The involution of this whorl is 

 about two fifths, and that of the ninth a trifle over one half. The peculiar flatten- 

 ing of the sides and form of the adult whorl, and the amount of involution, are 

 close approximations to the adult characteristics of Amm. Charmassei, but the 

 septa are different and the young more robust; the pilas are developed earlier 

 and more rapidly, and the abdominal channel also. In some specimens, however, 

 these last are not noticeable until quite a late period, the pila? being continuous 

 across the abdomen, as in Der. planicostum, even on the sixth volution. 



In the collections at the Stuttgardt Museum are several very fine specimens 

 of the old age of this species, and it is easy to distinguish it from Charmassei by 

 the narrowness of the whorls and its more open umbilicus and discoidal aspect. 

 One of the largest of these measures 495 mm., the last whorl 17 mm. ; another 

 measures 515 mm., and the last whorl 18.5 mm. 



In the Museum at Stuttgardt, in the centre of a crushed specimen of the true 

 angulata from Kirchheim, the young was very clearly exposed. This had very 

 smooth and round, though rather stout whorls. The pilaa appeared on the sides 

 as faint folds, which are straight at first, then curve, reach the abdomen, and 

 finally cross it with a forward inflection. These become very prominent and 

 decided before the channel is formed, which finally cuts through the pilse. This 

 variation, however, is considerable, since in the adult of this specimen the channel 

 is only partially developed, the pilse being only about half cut through, though 

 the specimen is about two and one fourth inches in diameter. There is here a 

 close likeness to some of the trias forms, but not to the true planorbe, which the 

 young does not resemble closely. They resemble Wsehneroceras very closely. 

 The same relations were observed in young specimens in Professor Quenstedt's 

 collection, and in the Museum of Comparative Zoology. 



It often occurs, also, that after the channel is developed, and the shell is 

 quite large, the pilse again cross the abdomen, but this . is not so frequent as 

 has been supposed. They oftener remain separate until old age. 



The original of Amm. angulatus, Sow., which we saw in the British Museum, is 

 only a malformed communis} 



1 See also Wright, Lias Amm , p. 473. 



