212 GENESIS OF THE ARIETIDA. 
trast with the greater involution and flatness of the shell of denotutus. On the 
other hand, denolatus as figured by Wright, when compared with the accurate 
figure of adult Codlenoti by D’Orbigny and of the young by Dumortier, shows that 
the latter is a form which is smooth, acute, and geratologous to an extreme 
degree at a much earlier age than either denotatus or wmnpendens. 
Asteroceras Collenoti, Hyarr. 
Plate IX. Fig. 10-11 b. Plate X. Fig. 10. Summ. PI. XIII. Fig. 5. 
Amm. Collenoti, D’OrB., Terr. Jurass., I. p. 395, pl. xev. 
Aster. Collenotit, Hyatt, Bull. Mus. Comp. Zool., I., No. 5, p. 80. 
Amm. Cluniacensis, DumMort, Etud. Pal. Bassin du Rhone, p. 148, pl. xxv. fig. 8-10. 
Aigoc. Slatteri, Wrieur, Lias Amm., p. 374, pl. 1. fig. 6-8 (not fig. 1-5). 
Localities. — St. Thibault, Semur. 
D’Orbigny’s two original specimens now in the Museum of Comparative 
Zovlogy show that his figures are faithful. The pile in a specimen from St. 
Thibault, figured on Plate IX. Fig. 10-11 of this memoir, arise as lateral folds on 
the last quarter of the second volution, and become fully developed on the third 
whorl. On the first quarter of the third whorl, the keel appears, and on the second 
quarter the channels, but these are at first only linear depressions. At this stage 
the involution is about one third. On the second quarter of the fourth whorl, the 
channels are still shallow, but have depressed, lateral, entire ridges, the pile being 
prominent near the dorsum and disappearing on the edge of the abdomen before 
reaching the channel ridges. The involution is now about one half. The transi- 
tion from the rounded whorl of the early nealogic stage to the acute form char- 
acteristic of the species takes place with extraordinary rapidity during the first 
quarter of the third volution. The usual intermediate stage, common in other 
species having a quadragonal or stout whorl with more or less depressed abdomen 
and flattened parallel sides, is entirely omitted. During the fourth whorl the 
smooth inflected zones, which represent the channels in this species, become - 
developed, and the pile are better defined. On the first quarter of the fifth 
volution the elevation and sharpness of the abdomen increase, but no obvious 
changes occur in other characters; the involution of the whorl exceeds one half 
of the side of the preceding whorl. At this stage and immediately preceding 
it the resemblance to the adult of denotatus is extremely close, except of course 
as regards the difference of size and the superior sharpness of the abdomen. On 
the last part of fifth volution degenerative changes begin, the pile rapidly disap- 
pear, and the channel zones become less distinct.’ 
In D’Orbigny’s other specimen, from Champlony near Semur, the pil are 
equally obsolescent on the last part of the third whorl, and on the first quarter 
of the fourth they are represented but faintly on the sides. 
In another specimen, figured on Plate X. Fig. 10,7 from the same locality, the 
pile are entirely absent, being represented only by lines of shell growth, or 
hardly perceptible folds, even on the first part of the fifth volution. On the 
1 Figured by D’Orbigny, Terr. Jurass., pl. xev. fig. 6, 7. 
2 Also figured by the same author on pl. xev. fig. 8. 
