208 GENESIS OF THE ARIETID^. 



Geometricus bed, and one, broken across, from the Obtusus bed at Balingen. 

 The last (PI. X. Fig. 3) shows the young with shallower channels and flattened 

 sides, somewhat similar to Ast. obtusum at an older period. 1 The sides of the 

 young, however, are flatter and broader than I have yet seen in any variety of 

 stellare, and more like those of an aged Cor. trigonatum. The characteristics of 

 the nealogic stages and of the adult are evidently geratologous. This spe- 

 cimen is 258 mm. in diameter. The whorl of the adult is smooth, and of same 

 shape as the senile stage of Ast. obtusum, and the sutures also agree exactly with 

 those of the old of that species. Comparing it with a specimen of Turned from 

 Endigen of about the same size, it is seen that the latter has a much broader 

 abdomen, deeper channels, more trenchant keel, and keeps its pilee and form 

 intact until a much later age, than Ast. acceleratum, and it does not increase in 

 size so fast, is not so involute, and grows much larger. There are also several 

 very fine specimens from Endigen, which enable lis to connect acceleratum with 

 stellare without the aid of the similar varieties from Semur or Gmiind, and they 

 are regarded as so connected by Professor Fraas, who has labelled them A mm. 

 obtusus. There is a series of five perfect specimens belonging to the Stuttgardt 

 Museum, sufficient to convince the most sceptical. Nevertheless, it differs in the 

 form of the young and in the adult from either stellare or obtusum, being both 

 more involute and more accelerated in its mode of development. It is, as a rule, 

 identified with BrooJd, and it bears the same relation to obtusum that Ast. impendens 

 bears to Turner/. 



Second Subseries. 

 Asteroceras Turneri, Hyatt. 



Plate IX. Fig. 8, 9. Somm. PI. XIII. Fig. 3. 



Amm. Turneri, Sow., Mia. Conch., V. p. 75, pi. cccclii. 



Aster, stellare, Hyatt, Ball. Mus. Corap. Zool., I., No. V. p. 80. 



.4mm. eompressaries, Quenst., Aram. Schwab. Jura, p. 126, pi. xvii. fig. 4-6. 



Amm. cf. obtusus, Quenst., Ibid., p. 143, pi. xix. fig. 9. 



Amm. umlaries, Quenst., Ibid., p. 148, pi. xx. fig. 2-6. 



Amm. Turneri, Quenst., Ibid., p. 142, pi. xix. fig. 5 (not fig. 6-8). 



Arietites Turneri, Wright, Lias Amm., p. 292, pi. xii. fig. 1-6. 



Localities. — Lyme Regis, Gloucester, Semur. 



The amount of involution on the third quarter of the sixth whorl is two fifths 

 of the side. The abdomen is broader, the channels more deeply sunken and 

 broader, and the keel thinner and less immature in its aspect, than in Ast. stellare 

 or Ast. acceleratum. The sides also are flatter, and the dorsum is but very slightly 

 broader than the abdomen. The umbilical shoulders are abrupt, but the umbili- 

 cus is hardly so deep as in acceleratum, the lateral increase of the shoulders by 

 growth being considerably less than in that species, though greater than in typical 

 varieties of obtusum. The pilse are thinner, more numerous, and do not have the 



1 This figure is erroneous in one important particular. The youngest part of the abdomen visible 

 in the figure, as seen from the front, is much flattened, whereas in the specimen it was but slightly more 

 depressed than in the older part of the same whorl, represented as trigonal immediately below. 



