64 GENESIS OF THE ARIETIDiE. 



From tins species we may follow two lines of evolution, one into Cor. trigona- 

 tum, 1 and one into Cor. Gnaicndense? The former, trigonatnm, can be distinguished 

 by its young whorls, which were stouter than is usual in the young of lyra, by 

 the increasing amount of the involution, which is no longer confined to the ab- 

 dominal region, but covers in the geniculos in some specimens, and by the earlier 

 period at which the pike become fold-like and the abdomen subangular. 



Cor. Gmuendense is also distinguishable from lyra and from trigonatum by its 

 extremely flattened whorls in the young and adult, though it may otherwise 

 exactly resemble the young of the typical variety of lyra. The involution did 

 not increase by growth, but was confined to the abdominal area, and limited 

 laterally by the geniculce. The senile changes were very distinct, and occurred 

 at earlier stages than in lyra; the abdomen 3 became narrower and the sides 

 convergent before the loss of the tubercles. Thus we can say with certainty, 

 that, in this species, degradational old age changes began to alter the form before 

 the other adult characteristics showed signs of obsolescence. 



The Second Subseries of Coroniceras begins again with Cor. kridion. The con- 

 nection is made by a very remarkable form, Cor. Scmseanum. The young in 

 some specimens are like the young of kridion, and then in the next older but still 

 immature stages 4 acquire the characteristics of the adults of kridion. The typical 

 Sanzeanum maintained until a late period of growth, and probably throughout 

 the ephebolic stages in some specimens, the broad abdomen and prominent tuber- 

 culated geniculce of the young, but the sides usually became slightly convergent. 5 

 Variety Gandryi, 6 for a greater or a less number of whorls repeated the typical 

 form of Sauzeanum. The geniculas in older stages were carried inwards, the 

 abdomen became slightly elevated and proportionately narrower, and the tuber- 

 cles almost obsolescent. A large whorl, over six centimeters in the abdomino- 

 dorsal diameter, was observed, in which these characteristics were not changed 

 otherwise than by the shallowing of the channels and the depression of the keel. 



The transition forms from variety Gandryi to Cor. bisidcatum 1 are not perfectly 

 satisfactory. They are, however, nearly allied by the peculiarities of the abdo- 

 men and geniculaa in the young of bisidcatus, which are similar to those of the 

 young of variety Gaudryi until a late stage of growth. Two specimens of large 

 size were observed. One had a diameter of 620 mm., the tubercles and channels 

 were almost obsolescent, the pilaa very thick and fold-like, but the geniculse were 

 well developed and prominent, as in the adult. Another, 650 mm. in diameter, 

 had the abdomen much narrower proportionately, the tubercles had disappeared, 

 and the channels were almost obsolete, the keel being much reduced in size. 

 The former was 170 mm. in the abdomino-dorsal diameter of the last whorl, but 

 the latter reached the enormous size of 240 mm. in the same part. There was, 

 therefore, a difference of 70 mm. in the diameter of the last whorl, as compared 

 with the difference of only 30 mm. in the diameter of the entire shell. 



i PI. vi. fig. 1, 2; PI. vii. fig. 1; Summ. PI. xii. fig. 15. 



2 PI. v. fig. 4-9; Summ. PI. xii. fig. 14. " » PI. v. fig. 6, 8-9. 



4 PI. vi. fig. 9, 10; Summ. PI. xii. fig. 10. 6 PI. vi. fig. 5, 6, 12, 13. 



6 PI. vi. fig. 14. 7 PI. vii. fig. 2-8; Summ. PI. xii. fig. 11. 



