FIFTH, OR AGASSICERAN BRANCH. 211 



Asteroceras impendens, Hyatt. 



Plate X. Fig. 6-9. 



Ariel, impendens, Wkight, Lias Amm., p. 302, pi. xxii. A, fig. 1-5. 

 Ariet. Collenoti, Ibid., p. 304, pi. xxii. A, fig. fi-9 ; pi. xxii. B, fig. 1-3. 

 Amm. Fowleri, Buckm., Mureh. Geol. Cheltenh., pi. xii.. fig. 7. 

 Amm. impendens, Quenst., Amm. Schwab. Jura, p. 151, pi. xx. fig. 7-10. 

 Ariet. impendens, Blake, Yorkshire Lias, pi. vi. fig. 7. 



Localities. — Semur, Lyme Regis. 



This species differs from Brooki in the greater amount of the involution, the 

 smaller size, the earlier age at which the same form of whorl is passed through, 

 and the earlier age at which degeneration begins. 



In Wright's "Lias Ammonites " the figures of Brooki, impendens, and Collenoti 

 show in the clearest manner what are the proper limits of the species. The fig- 

 ure of Brooki, on Plate VI. Fig. 4, is taken from the less involute form, which 

 retains its pilae until a. late stage of growth; that of Collenoti on Plate XXII. B 

 is an old and very large individual of impendens, which approximates to Brooki, but 

 has the usual difference in the amount of involution and begins to show degen- 

 eration of the pilae also earlier than is common in Brooki; that of Collenoti on 

 Plate XXII. A, Fig. 6-8, is a yet more accelerated form, growing old and losing 

 pilae, etc. earlier than in the specimen shown on Plate XXII. B. The figure of 

 impendens itself is still more accelerated than the specimen figured on Plate 

 XXII. A, Fig. 6-8, and has also somewhat stouter whorls. The young figured 

 on Plate XXII. A, Fig. 4, is similar to the adult of the true Ast. Brooki, having 

 the same form, pilae, and involution until a late nealogic stage. 



All of these forms have the broad abdomen, the peculiar channels, and the 

 young like Brooki, and are quite distinct from the true Ast. Collenoti. They are 

 undoubtedly transitional forms connecting the two species. Those who wish may 

 join them, but, as we have previously said, all the series and about all the species 

 of the Arietid3e are closely connected by intermediate forms and modifications, 

 and, to be really consistent, we must then also include the entire family under 

 a single specific name. We doubt if any paleontologist would secure serious 

 support if he attempted to do this. 



Asteroceras denotatum, Hyatt. 



Amm. denotatus, Simps., Foss. Yorkshire Lias, p. 76. 



Ariet. denotatus, Wiught, Lias Amm., pi. vi. fig. 1 (Collenoti in the text, p. 304). 



Amm. tenellus, Simps., Foss. Yorkshire Lias, p. 97V 



This species has been universally placed either with Brooki or Collenoti. It is, 

 however, quite distinct from either of these forms. If one compares the figure 

 of denotatus by Wright with the young of impendens figured by the same author on 

 Plate XXII. A, Fig. 4, and that of the young of the same species by Quenstedt, 

 Amm. Schwab. Jura, Plate XX. Fig. 8, it will be seen that denotatus stands just 

 between impendens and Collenoti. The young of impendens is much less involute, 

 and shows the same stout whorl as in the adult, a form which is in strong con- 



