68 GENESIS OF THE ARIETIDA. 
Second Subseries. —Some specimens of Amm. stellare, Sow. have a young 
stage during which the sides become more or less flattened and parallel. These 
are intermediate between true odtusum and Turneri. The old of the stout va- 
rieties of obtusum, and specimens of Twrneri in their first senile stage, have char- 
acteristics similar to those of the adults of stel/are, and occasion confusion in the 
identification of fragments. The extreme senile metamorphoses of Zwrneri, when 
the whorl became smooth, the channels shallow, the sides convergent, and the 
abdomen narrow, occurred, as in other species, at variable ages, sometimes in 
shells only 102 mm. in diameter. 
The differences between the adult of Twrneri and the adult of the next 
species, As?. Brooki,? are well marked in most specimens, but the stoutest and 
least involute forms of the latter are very closely allied to the former.. The adults 
of the stouter variety of Brooki retained the channels, the keel remained promi- 
nent, the sides remarkably flattened, and the pile in some specimens prominent 
and like those of Turneri, but the whorls were generally more involute. The 
young have very close resemblance to the adults of Turner’. The young? of 
Ast. impendens had no very close resemblances to Turner at any stage. They 
repeated the adult characteristics of the stouter variety of Brooki during the 
nealogic stages, and in the adults exaggerated the normal tendency to conver- 
gence of the sides, depression of the pile, and narrowing of the abdomen. The 
adult of this variety approximated quite closely to the senile stage of sfellare. 
The fine series of figures given by Wright in his “Lias Ammonites” shows 
most completely the transitional forms of dmpendens, and are referred to below 
in the description of Brooki. Ast. denotatum is simply a more involute form of 
impendens. 
The next species, Ast. Collenoti,s can be traced directly to the preceding 
form, and, if my translation of the facts is correct, it is the geratologous offspring 
of impendens or denotatum. The young? were similar to the young and adults of 
ampendens, and also more remotely to the adults of sted/are, but the next or first 
ephebolic stage was precisely similar in all respects, except the sutures, to the 
first senile stages of tmpendens. In the adults of one variety this stage retained 
distinct pil, though in other specimens the sides became smooth. The involu- 
tion of the adult whorl was more considerable than in unpendens, and the shell 
closer in this respect to denotatum. 
The extreme variety of Collenoti had a similar form and development, but 
was somewhat sharper on the abdomen, and the pilss were wholly confined to the 
nealogic stages, the adult stage being similar in form and characteristics, except 
in the sutures, to the extreme old age of impendens. Thus the characteristics 
of the transient senile stages of Ast. obfuswm and other normal species were 
similar to the permanent characters of the ephebolic and even nealogic stages 
of degenerate or pathological species like Ast. acceleratum and Ast. Collenoti. 
1 PTS aS, tee 2, Oe 2 Summ. Pl. xiii. fig. 4. 
ole. tip, 6-9, * Pl. x. fig. 10; Summ, Pl, xiii, fig. 5. 
5 Pl. ix. fig. 10-11 b. 
nan 
