180 GENESIS OF THE ARIETID.E. 



the channels are indicated by faint depressions. Upon the third quarter of this 

 whorl continuous channel ridges appeared, flanked by tuberculated geniculoa 

 bending forwards upon the abdomen. The flatness of the abdomen is due tq the 

 rise of the geniculoa upon the sides, which occurred previous to the appearance 

 of the tubercles. The tuberculated geniculoa 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 geniculoa. 



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 geniculoa. 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. brevidor sails, Plate VII., would have had comparatively short ab- 

 dominal lobes, to whatever species they might have belonged. 1 Quenstedt com- 

 plains of Wright for not paying attention to his distinctions, but we think 

 Wright's figures of multicostalus 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 



i For example, the huge Cor. Bucklandi, Amm. Schwab. Jura, pi. ix. fig. 1, and coronaries, pi. 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. 



