64 GENESIS OF THE ARIETIDA, 
From this species we may follow two lines of evolution, one into Cor. trigona- 
tum,’ and one into Cor. Gimuendense” The former, trigonatum, can be distinguished 
by its young whorls, which were stouter than is usual in the young of Wra, by 
the increasing amount of the involution, which is no longer confined to the ab- 
dominal region, but covers in the geniculx in some specimens, and by the earlier 
period at which the pile become fold-like and the abdomen subangular. 
For. Gmuendense is also distinguishable from dyra 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 (ra. The involution did 
not increase by growth, but was confined to the abdominal area, and limited 
laterally by the genicule. The senile changes were very distinct, and occurred 
at earlier stages than in lyra; the abdomen® 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. Sauzeanum. The young in 
some specimens are like the young of éridion, and then in the next older but still 
immature stages* acquire the characteristics of the adults of kridion. The typical 
Sauzeanum maintained until a late period of growth, and probably throughout 
the ephebolic stages in some specimens, the broad abdomen and prominent tuber- 
culated geniculz of the young, but the sides usually became slightly convergent.’ 
Variety Gaudryi,° for a greater or a less number of whorls repeated the typical 
form of Sauzeanum. The geniculz 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 Gaudryi to Cor. bistilcatum™ are not perfectly 
satisfactory. They are, however, nearly allied by the peculiarities of the abdo- 
men and genicule in the young of dsulcatus, 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 pila very thick and fold-like, but the genicule 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. 
1 Bh. vi. oo, 1,2; PL vi. figs 1; Summ. Pl. xu, fig? V5: 
2 Pl. v. fig. 4-9; Summ, Pl. xii. fig. 14. ’ Pl. v. fig. 6, 8-9. 
4 Pl. vi. fio. 0, 10; Samm. Pl. xit. fig..10. oO Vite. 0, 0) 12, Lo. 
OOPL Vi tig id. as T Pl. vii. fig. 2-8; Summ. Pl. xii fig. 11. 
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