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GENEALOGY. 67 
the divergent-sided whorl is noticeable. This is often replaced by a parallel-sided 
whorl on the fourth volution, and this in its turn is replaced by the convergent 
sides of the fifth whorl? The divergent-sided whorl, with its tuberculated pile, 
is skipped in the development of some specimens of obéwsum, and it is replaced on 
the third whorl by the parallel-sided smooth whorl and pile of the later stage, 
instead of on the fourth whorl, as described above.® 
The broad abdomen and the correlative divergent-sided larval form of Ast. 
obtusum are retained in the adults of some varieties,t but even in these the pile 
are smooth and without genicule, and the whorls discoidal. Notwithstand- 
ing this fact and the enormous size reached by some normal specimens before 
manifesting old age, there are specimens in the closely allied Ast. stellare and 
acecleratum which exhibit a remarkable tendency to assume retrogressive char- 
acteristics, and to inherit them in their younger stages, while still becoming 
more involute and holding the keel comparatively unchanged. These characters 
induced me at first to estimate the whole series as geratologous, but this view 
cannot be maintained. Most specimens lose the tubercles early, or do not have 
them at all, the pila become mere folds, and bend forward, the keel being low 
and broad and the channels shallow. There is so close a resemblance between 
these ephebolic characteristics and the old age stages of the common English 
form of Bucklandi at Lyme Regis, especially in the stout varieties of oblusum, 
that collectors frequently call the old of Bucklandi by the name of Amm. obtusus. 
Upon one oceasion I was myself completely deceived by the exposed portion of 
a whorl, which, when finally cleared of its surrounding matrix, was readily iden- 
tified as a senile specimen of typical Bucklandi. Nevertheless the characteristics 
of Ast. obtusum, when compared with the radical Agas. striartes or levigatum, are not 
geratologous, but nealogic. They have the same relation to the characteristics of 
these radical forms that the fold-like pile and immature whorls of Cal. John- 
ston’ have to those of its immediate radical, Psi. planorbe. Their real value as 
radical characters is shown also by the fact, that in some full grown specimens of 
oblusum tubercles appear, and in the ephebolic stages of Ast. Turnert of the next 
subseries the typical arietian characters appear, namely, deep channels, well 
defined keel, and quadragonal form. These therefore occur in the same succes- 
sion as in other series of the Arietida, and during the growth of the individual 
they appear in similar order. 
Ast. acceleratum, the second and last of this subseries, occurs rarely, but is 
found in several collections. It has young until a late period precisely identical 
with the young of certain varieties of obtusum, and the adults of stellare. This 
stage, which may last until the individuals are from 76 to 89 mm. in diameter, 
is immediately followed by a stage in which the involution is increased, the sides 
are flattened, the abdomen narrowed, and the pile obsolescent. In fact, during 
its adult stage a form and characteristics are produced very similar to the stouter 
varieties of Brooki, with which it was at first associated. 
1 Pl. viii. fig. 6. 2 Pl, vai ae, 3 
8 PJ, viii. fig. 8, does not show the inner whorls accurately enough, and a ecmparison of the figures is 
necessary in order to give an accurate idea of the development. 
4 Summ. PI. xiii. fig. 2, has parallel sides, but belongs to this gibbous whorled variety. 
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