SIXTH, OR OXYNOTICERAN BRANCH. 215 
stouter varieties of oxynotum. On the other hand, the duration of the s¢riaries- 
like stage in Greenoughi, the sutures of the adult, which are simpler or more 
arietian in outline than in oxynotum, and the essentially hollow keel, seem to indi- 
cate an independent origin directly from Agas. striaries. The specimen of Gree- 
nought in the Museum of Stuttgardt, in which the hollow keel was observed, had 
the outer shell of the keel remarkably thick, but the interior evidently hollow, 
while in the French specimens at Semur the shell of the keel was of usual thick- 
ness. The former may possibly indicate a transition to oxynotwn. 
Oxyn. Giubali, however, by the resemblances of the young to the adult of 
Greenough, and by the younger period at which the adult characteristics of. the 
species are inherited, is apparently a direct derivative from Greenough. The same 
reasons would also apply to Lotharingum, in which the young lose the sfriaries-like 
stage almost entirely, and repeat only the adult form and characteristics of Gree- 
nough. These characteristics, the continual decrease in the duration of the adult 
stages, and the earlier period at which the senile stage of decline makes its ap- 
pearance in each successive species, indicate that these three species belong to a 
distinct subseries from oxynotum. The earliest representatives are found in the 
Tuberculatus bed at Semur, and in the Oxynotus bed of Dumortier in the basin of 
the Rhone. The different species seem, therefore, to form two subseries; one 
consisting of oxynotum and its allies, and another composed of Greenoughi, with 
Guibali and Lotharingum and their allies, but all on the same geological horizons. 
First SUBSERIES. 
Oxynoticeras oxynotum, Hyarr. 
Plate X. Fig. 4, 5, 14-22, 27. Summ. Pl. XIII. Fig. 9, 10. 
Amm. orynotus, QuENsT., Petrefactkunde, XCVIIL., pl. v. fig. 11; Amm. Schwiib. Jura, pl. xxii. fig. 28-49. 
Amaltheus oxynotus, Wriegut, Lias Amm., pl. xlvi. fig. 4-6. 
Amm. oxynotus, Dum., Etud. Pal. Bassin du Rhone, pl. xxxiii. fig. 2, 3, 5 (not fis; 2). 
Localities. — Lyme Regis, Cheltenham, Stonehouse, Gloucester, Hanover, Pliensbach, Balingen, Salins. 
The young are at first smooth, and in this early stage resemble closely Psilo- 
ceras. They may continue to retain this smoothness and the rotundity of the 
abdomen until the specimen is 12 mm. in diameter, as figured on Plate X. Fig. 17. 
Their sides then become flatter, and slight folds and striations appear. The re- 
semblance to Agas. striaries is so decided in some young specimens, that, if found 
independently, no one would hesitate to place them in the same genus as closely 
allied species. This same resemblance also necessarily includes a close likeness 
in the young to Psil. planorbe. The sutures are also similar to those of Agas. 
striaries, as may be seen in collections at Stuttgardt, where there is a very fine 
series of exceptional varieties collected by Professor Fraas. 
The keel appears much later in the striaries-like forms figured on Plate X. 
Fig. 4, 5, than in the normal forms figured on Plate X. Fig. 16-18, but in all 
varieties the arietian characteristics of the sutures are apparent. The keel on 
its first appearance seems to be solid, though I could not, as in the case of 
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