Hyatt.] 20 [May 20, 



The minor lobes are more numerous, deeper, and pointed than in 

 the Arietidae, the minor cells being quite leaf-like, the abdominal lobe 

 considerably shallower than the superior laterals, the inferior laterals 

 very short, and the auxiliary lobes quite numerous and bending poste- 

 riorly at a considerable angle. The seventh whorl increases in size 

 with great rapidity, the abdomen becoming narrower, the channel 

 shallower, the pila? 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 flattening 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 pila? are de- 

 veloped < arlier and more rapidly, and the abdominal channel alsD 

 In some specimens, however, these last are not noticeable until quite 

 a late period, the pilas being continuous across the abdomen, as in 

 D. planicostum, even on the sixth volution. 



In the collections at the Stuttgart 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 angulatus 

 measures 495 mm., the last whorl 17 mm.; another measures 515 mm., 

 and last whorl 18.5 mm. 



In the Museum at Stuttgart, in the centre of a crushed specimen 

 of the true angulatus from Kirchheim, the young was very clearly 

 exposed. This had very smooth and round, , though rather stout 

 whorls. The pilse 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 de- 

 cided before the channel is formed, which finally cuts through the 

 pilse. This variety, however, is considerable, since in the adult of 

 this specimen the channel is only partially developed, the pilae 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 Planorbis which the young 

 does not resemble at all. 



In young specimens in Prof. Quenstedt's collection, and the Mu- 

 seum of Comparative .Zoology, the same was observed. It often occurs 

 also that after the character is developed, and the shell quite large, 



