190 GENESIS OF THE ARIETID^E. 



than any other observed, and the geniculae 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 pilse and geniculte 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 piloa 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 

 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 

 the latter part of the fourth whorl they are about even (Plate III. Fig. 23 a). The 

 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 

 in addition the following : — 



1. Forms which at a comparatively early age have an abdomen and tubercles 

 like the adult of var. Gaudryi of Cor. Saiizeanwn. 



2. Three specimens with young, having the typical broad abdomen of latum, 

 but speedily changed by growth so as to resemble the young of Cor. orbiculatmn. 



3. One specimen has sides flattened, pilee numerous and single, approximat- 

 ing to the adult Bucklandi in aspect. 



4. Another has single pilse in the young and double in the adult, just the 

 reverse of all other specimens yet observed. 



5. Another has the double pilse 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. 

 latum. 



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 III., 

 the similarity of the sutures of the young of rotiforme, Plate III. Fig. 10 a, and 

 of latum, 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. latum. 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. 

 Bucklandi. The sutures of Quenstedt's Amm. Bucklandi pinguis agree remarkably 



