212 GENESIS OF THE ARIETID.E. 



trast with the greater involution and flatness of the shell of denotatus. On the 

 other hand, denolaius as figured by Wright, when compared with the accurate 

 fio-ure of adult Collenoliby 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 impendens. 



Asteroceras Collenoti, Htatt. 



Plate IX. Fig. 10-11 b. Plate X. Fig. 10. Summ PI. XIII. Fig 5. 



Amm. Collenoti, D'Orb., Terr. Jurass., I. p. 395, pi. xcv. 

 Aster. Collenoti, Hyatt, Bull. Mus. Comp. Zool., I., No. 5, p. 80. 

 Amm. Cluniacensis, Dumort, Etud. Pal. Bassin du Rhone, p. 14S, pi. xxv. fig. 8-10. 

 JEgoc. Slatteri, Wright, Lias Amm., p 374, pi. 1. fig. 6-S (not fig. 1-5). 

 Localities. — St. Tliibault, Semur. 



D'Orbiuny's two original specimens now in the Museum of Comparative 

 Zoology show that his figures are faithful. The pilse 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 pilaa 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 entirety omitted. During the fourth whorl the 

 smooth inflected zones, which represent the channels in this species, become 

 developed, and the pilaa 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 pilte rapidly disap- 

 pear, and the channel zones become less distinct. 1 



In D'Orbiguy's other specimen, from Champlony near Semur, the pila? 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, 2 from the same locality, the 

 pilse 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 , pi. xcv. fig. 6, 7. 



2 Also figured by the same author on pi. xcv. fig. 8. 



