GENEALOGY. 67 



the divergent-sided whorl is noticeable. 1 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. 2 The divergent-sided whorl, with its tuberculated pilse, 

 is skipped in the development of some specimens of obtusum, and it is replaced on 

 the third whorl by the parallel-sided smooth whorl and pike of the later stage, 

 instead of on the fourth whorl, as described above. 3 



The broad abdomen and the correlative divergent-sided larval form of^ls^. 

 obtusum are retained in the adults of some varieties, 4 but even in these the pilae 

 are smooth and without geniculae, 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 

 acceleration 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 pilse 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 BucMandi at Lyme Regis, especially in the stout varieties of obtusum, 

 that collectors frequently call the old of BucMandi by the name of Amm. obhtsus. 

 Upon one occasion 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 BucMandi. Nevertheless the characteristics 

 of Ast. obtusum, when compared with the radical Agas. striaries or Icevigalum, are not 

 geratologous, but nealogic. They have the same relation to the characteristics of 

 these radical forms that the fold-like pilse and immature whorls of Gal. John- 

 stoni have to those of its immediate radical, Psil. planorbe. Their real value as 

 radical characters is shown also by the fact, that in some full grown specimens of 

 obtusum tubercles appear, and in the ephebolic stages of Ast. Turneri 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 Arietidse, and during the growth of the individual 

 they appear in similar order. 



Ast. acceleration, 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 pike 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 PI. viii. fig. 8. 



3 PI. viii. fig. 8, does not show the inner whorls accurately enough, and a comparison of the figures is 

 necessary in order to give an accurate idea of the development. 



* Summ. PI. xiii. fig. 2, has parallel sides, but belongs to this gibbous whorled variety. 



