SS 
190 GENESIS OF THE ARIETIDA. 
than any other observed, and the genicule were hardly yet perceptible on the 
second quarter of the fourth volution. Another cast, though nearly as broad as 
the last, is somewhat flattened on either side of the keel; it has prominent and 
immature tubercles, though the pile and genicule are still imperceptible, and the ' 
channels still undeveloped on the latter part of the fourth volution. These two 
specimens show the retention of nealogic form and characters. When specimens 
, develop more rapidly, the great breadth of the abdomen’ begins to decrease, and ' 
true pile replace the tubercular folds on the fourth whorl. 
The sutures in this species are peculiar in respect to the great breadth of the 
lobes and saddles in comparison with their length or depth, the regularity and é 
small size of the marginal lobes, and the rotundity of the marginal saddles even 
in the oldest stage examined, on the second quarter of the fifth whorl. At this 
| time the superior lateral saddles are not so deep as the inferior laterals, but in 
1 the latter part of the fourth whorl they are about even (Plate Ill. Fig. 23a). The 
1 abdominal lobe exceeds the superior laterals by less than one third, and the 
inferior lateral lobes exceed the superior laterals in the same proportion. At the | 
earliest period examined, the first quarter of the fourth whorl, the abdominal lobe 
is only about one fourth longer than the superior laterals, and the inferior lateral 
saddles, instead of being deeper, are about one third shallower than the superior | 
) 
| 
laterals. 
A splendid series of this species from the Bucklandi bed at Semur gives very 
remarkable variations. The young have all the variations above described, and 
1 in addition the following : — 
i 1. Forms which at a comparatively early age have an abdomen and tubercles 
like the adult of var. Gaudryi of Cor. Sauzeanum. 
2. Three specimens with young, having the typical broad abdomen of datum, 
but speedily changed by growth so as to resemble the young of Cor, orbiculatum. 
3. One specimen has sides flattened, pile numerous and single, approximat- 
1 ing to the adult Bucklandi in aspect. ; 
| 4. Another has single pile in the young and double in the adult, just the 
1 reverse of all other specimens yet observed. 
5. Another has the double pilx in the young, but the sides are gibbous in- 
stead of being flat or divergent, and it then speedily acquires convergent sides, 
becoming similar to Cor. rotiforme. The abdomino-dorsal diameter is less than 
usual, and the increase in size more gradual than in the typical form of Cor. 
latuin. 
‘These variations and resemblances all seem to be expressions of transient 
tendencies, except those which approximate to Cor. kridion and rotiforme. The 
agreement of the young forms as shown above, and of Fig. 15 and 21, Plate IIL, 
the similarity of the sutures of the young of rotiforme, Plate III. Fig. 10 a, and 
he: aes 
1 of datum, Fig. 23 a, show that the differences between these forms are not. great, 
and consist mostly in the excessive development of the broad-abdomened whorl 
in some varieties of Cor. datum. All the specimens I have seen are also of small 
size, and seem to be the young of some yet unknown adult form similar to Cor. 
Buckland. The sutures of Quenstedt’s Amm. Bucklandi pinguis agree remarkably 
