168 GENESIS OF THE ARIETIDZ. 
Arnioceras tardecrescens, Hyarr. 
Plate Il. Fig. 19, 21-22. Summ. Pl. XII. Fig. 6. 
Amm. tardecrescens, Hyarr, Bull. Mus. Comp. Zodl., I., No. 5, p. 74. 
Amm. tardecrescens, HAvER, Ceph. Lias Nordéstl. Alpen, p. 20, pl. iii. 
Amm. falcaries (pars), Quens'., Der Jura, pl. vii. fig. 7 (not fig. 6). 
Amm. falcaries densicosta, Quunsr., Amm. Schwab. Jura, pl. xiii. fig. 7. 
Localities. — Yorkshire, Semur, Durrenburg. 
The pile appear on the fourth whorl at variable times. There are fewer 
whorls, and they are wider from the abdomen to the dorsum and have generally 
rounder sides, than in Arn. Hartman. 
The abdominal lobe is equal to or somewhat longer than the superior laterals ; 
the inferior lateral saddles are shallower than the superior laterals, and the inferior 
lateral lobes are much shorter, sometimes a third less, than the superior laterals 
on the last quarter of the sixth volution. On the third quarter of the same 
whorl in the same specimen from Semur, the abdominal lobe was one half shorter 
than the superior laterals. The siphonal saddle was very large, and the superior 
lateral lobes very long and broad, with straight sides, the inferior lateral lobes two 
fifths shorter than the superior laterals. The first auxiliary saddles are visible 
on the sides. The marginal lobes are still hardly more than mere serrations, 
except along the bases of the saddles. The superior lateral lobes have very 
broad and minutely serrated summits. 
A specimen in the Museum of Stuttgardt had curious characteristics. The 
sutures are undoubtedly arnioceran, the keel very prominent, and the channels 
shallow. The form of the whorls and the pile, however, are similar to those of 
Conybeari. Another specimen from Boihingen is precisely similar, but the pile 
terminate abruptly at the genicule somewhat below the edges of the channels, 
instead of being continued upwards and forwards, as in the former and in Cony- 
beari. Neither of these shows the young whorls well enough to enable one to 
identify them accurately either with Arn. Hartmann’ or Arn. tardecrescens, but 
they are undoubtedly arnioceran forms. Both are referred to the Geometricus 
zone. 
The original of Quenstedt’s figure in Der Jura, from Pforen, Baden, named 
Amm. falcaries, has the. narrow sulcated keeled abdomen, rounded sides, and 
pile of this species. There is also from Achdorf in Baden a specimen about 
26 mm. in diameter, which just begins to show the pile.’ The figure by 
Quenstedt? shows a form in which the young is smooth for a considerable num- 
ber of whorls. 
1 This may be the specimen figured in Amm. Schwab. Jura, pl. xiii. fig. 18, as Amm. falcaries levis- 
simus. 
2 Amm. Schwab, Jura, pl. xiii. fig. 7. 
