170 GENESIS OF THE ARIETIRE. 



Varieties B and C, from Semur, showed tubercular-like folds on the latter 

 part of the first, and part of the second whorl, which were continued on the um- 

 bilical border of the succeeding whorls, gradually developing into true pilae on the 

 third whorl. 



For a short interval in the specimen of variety A the shell is smooth again, 

 and in that of variety C the folds still remain apparent, but so depressed that 

 they were made out with difficult}'. On the first quarter of the third whorl in 

 one specimen of variety B, and the third quarter of a specimen of variety C, the 

 folds reappear on the umbilical border, but develop so gradually that lateral pilae 

 are not produced until the latter part of the first quarter of the fourth whorl. 

 Several other specimens of these same varieties, however, from the same locality, 

 did not show the psiloceran folds in the 3-oung, or differ from those of variety A. 



A fragment from Ra inert in the Museum of Stuttgardt is the true ceraiitoides 

 of Quenstedt, which differs from ceras in having a more prominent, narrower 

 abdomen and shallower channels. 



A specimen of the stout variety from Cheltenham, named Bocllryi, Bruck., has 

 well preserved young which show this species to be closely allied to ceras. The 

 thinner variety described above has the young much compressed, and similar to 

 the adult of the Cheltenham specimen. On the other hand, the young of the 

 Cheltenham specimen is precisely like the adult of Am. Hartmanni. Oppel's de- 

 scription shows that he identified the compressed variety as the Bodlei/i of Bruck., 

 and the stout varieties as Amm. geometricus. 



Arnioceras falcaries, Hyatt. 



Plate II. Fig. 35-37. 



Amm. falcaries, Quenst., Der Jura, p. 70, pi. vii. fig. 6 (not fig. 7). 

 A nun. falcaries, Qcenst., Amm. Schwab. Jura, pi. xiii fig. 12-14. 

 Arnioceras incipiens, Hyatt, Bull. Miis. Comp. Zool. I., No. 5, p. 74. 

 Amm. acuticarinatus, Simps.. Museum at Whitby'.' 1 

 Amm. Youngi, Simps.. Mod. Amm , p. 40 ? 



Localities. — Semur, Robin Hood's Bay, Baliugen. 



The sides are convex, the whorls compressed, and the abdomen obtusely 

 angular, keel prominent. The pila? begin with a line of tubercles, which appear 

 on the first half of the fourth whorl, preceding the true pilae by about one fourth 

 of a whorl. The pike are not strongly developed upon the umbilical shoulders 

 of the whorls, which in many specimens are almost smooth. 



Variety A, figured on Plate II. Fig. 25, 26, has prominent geniculce and 

 keel without channels, but some specimens leading to the next variety have less 

 prominent genicular. 



Variety B has less prominent pike and keel. The channels, though mere 

 linear depressions, begin to appear in some specimens. 



Variety C, figured on Plate II. Fig. 27, has even less prominent pilse, but a 

 keel with distinct narrow channels, and in some specimens the pilae were devel- 

 oped abruptly, not being preceded by the usual line of tubercles. 



1 The ? in these cases is due to the fact that 1 was not permitted to examine the originals in the 

 Museum at Whitby. 



