180 GENESIS OF THE ARIETID. 
the channels are indicated by faint depressions. Upon the third quarter of this 
whorl continuous channel ridges appeared, flanked by tuberculated geniculse 
bending forwards upon the abdomen. ‘The flatness of the abdomen is due to the 
rise of the geniculw upon the sides, which occurred previous to the appearance 
of the tubercles. The tuberculated genicule began to fall below the level of the 
abdomen upon the last quarter of the fourth whorl, and the abdomen became in 
consequence more elevated. The whorl acquired rapidly the flattened sides and 
peculiar aspect of the adult, and the transformation was completed on the third 
quarter of the fifth volution. Thus in the earlier stages there is a smooth ele- 
vated abdomen, which becomes flattened in the next stage, and then elevated 
again, but in the last stage it is furnished with keel, channels, and genicul. 
The sutures on the last part of the third whorl have an abdominal lobe one 
third longer than the superior laterals, but the superior and inferior lateral sad- 
dles are of equal depth. These proportions are still unchanged on the first half 
of the fifth whorl in some specimens, whereas in others, on the last half of the 
same whorl, the difference between the lobes and saddles is about one fifth, and 
on the second quarter of the sixth volution the proportion becomes one third. 
Before the close of the fourth volution the lobes become more complicated and 
broader at the top, and the marginal saddles more numerous and leaf-like, as in 
the adult of their own species; the inferior lateral saddles are also deeply cut by 
two marginal lobes into three marginal saddles. The similar tripartite division 
of the superior lateral saddles and superior lateral lobes becomes at the same 
time very marked. The saddles also broaden out at their bases, and upon the 
sixth volution spread over the genicula. This broadening of the bases of the 
saddles, and the position occupied by the bases of the superior lateral lobes in 
consequence of this, are characteristics of some value in the species. They also 
show that the external characteristics of the shell develop contemporaneously 
with the sutures, and arrive together at their adult development upon the latter 
part of the fifth whorl. The abdominal lobe extends beyond the superior lat- 
erals by about one third of its own length, and the inferior lateral saddles are 
deeper than the superior laterals in the same proportion, The abdominal lobe of 
the last suture of the third volution is one third longer than the superior lateral 
lobes, but the two larger saddles are of equal depth. Quenstedt describes and 
figures all specimens from Suabia as having an abdominal lobe shorter than the 
superior laterals, and the superior lateral lobes pointed. This character was found 
in the nealogic and adult stages. The figures of the very aged forms which he 
calls Amm. brevidorsalis, Plate VII., would have had comparatively short ab- 
dominal lobes, to whatever species they might have belonged.’ Quenstedt com- 
plains of Wright for not paying attention to his distinctions, but we think 
Wright’s figures of multicostatus show that he was right. ‘The English specimens 
had long abdominal lobes, and, like French and German shells described above, 
are undoubtedly identical with those figured by Quenstedt. in every other 
1 For example, the huge Cor. Bucklandi, Amm. Schwab. Jura, pl. ix. fig. 1, and coronaries, pl. xvi., 
of Quenstedt. Compare also the gradual decline in length of the abdominal lobe of an aged Cor. trigona- 
tum, Plate VII. of this monograph. 
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