168 GENESIS OF THE ARTETID^E. 



Arnioceras tardecrescens, Hyatt. 



Plate II. Fig. 19, 21-23. Sum in. PI. XII. Fig. G. 



Amm. tardecrescens, Hyatt, Bull. Mus. Comp. Zobl., I., No. 5, p. 74. 

 Aram, tardecrescens, Hauek, Ceph. Lias Nordostl. Alpen, p. 20, pi. iii. 

 Amm. falcaries (pars), Quknst., Der Jura, pi. vii. fig. 7 (not fig. 6). 

 Amm. falcaries densicusta, Quenst., Amm. Schwab. Jura, pi. xiii. fig. 7. 



Localities. — Yorkshire, Semur, Durrenburg. 



The pilas 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 Am. Hartmanni. 



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 pilae, however, are similar to those of 

 Conyheari. Another specimen from Boihingen is precisely similar, but the pilse 

 terminate abruptly at the geniculse somewhat below the edges of the channels, 

 instead of being continued upwards and forwards, as in the former and in Cony- 

 heari. Neither of these shows the young whorls well enough to enable one to 

 identify them accurately either with Am. Hartmanni or Am. 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 

 pila? of this species. There is also from Achdorf in Baden a specimen about 

 26 mm. in diameter, which just begins to show the pila?. 1 The figure by 

 Quenstedt 2 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, pi. xiii. fig. 18, as Amm. falcaries levis- 

 simus. 



2 Amm. Schwab. Jura, pi. xiii. fig. 7. 



