206 GENESIS OF THE ARIETID.E. 



pressure. The keel is low and broad, as in obtumm, though prominent enough 

 to show above the lateral ridges when seen from the side. D'Orbigny's figure of 

 obtusion is correct, and gives this variety as it occurs at Semur, the abdomen 

 sometimes being wider than the dorsum, instead of being narrower than or 

 about equal to the dorsum, as in all other varieties. 



There is a normal specimen of this species in the Museum at Stuttgardt, col- 

 lected in the Upper Bucklandi bed at Goppingen. The majority of specimens, 

 however, belong to the Obtusus bed just above this. 



Confusion, as remarked above, is not infrequently occasioned by the resem- 

 blances of the senile stages in Cor. Bucklandi to the adults of obtusion and BrooM. 

 Sometimes even old specimens of Cor. irigomtum and Gmuendense are mistaken 

 by paleontolongists of experience and reputation for obtusion, especially if the 

 inner whorls are concealed. The larger and older specimens of obtusion are apt 

 to acquire the more abrupt umbilical shoulders which are characteristic of the 

 young and adult of Ast. steUare. 



Asteroceras stellare, Hyatt. 



Plate IX. Fig. 3, 3. Plate X. Fig. 1, 2. 



Amm. stellaris, Sow., Min. Conch., I. p. 211, pi. xciii. 



Amm. stellaris, Ziet., Verst. Wuit , p. 15, pi. ii. fig. 5. 



Amm. Brooki, Quexst., Amm Schwab. Jura, p. 116, pi. xv. fig. 2, 3 (not pi. xx., xxi ). 



Ariet. Brooki, Wright, Lias Amm., p. 295, pi. xxii. fig. 1-6. 



Amm. obtusus suevicus, Quenst., Amm. Schwab. Jura, pi. xx. fig. 1. 



Localities. — Semur, Lyme Regis, Gmiind, Tubingen. 



The nealogic stages are similar to the older ephebolic stages of var. E of Ast. 

 obtusion. In point of fact, the two forms run into each other by intermediate 

 varieties, and can be separated only by artificial lines. The young in one 

 specimen from Whitby exhibited on the first quarter of the fifth volution 

 channels as deep, keel as prominent, and an abdomen considerably narrower 

 than those of the most highly modified forms of Ast. obtusion, var. E, on the last 

 quarter of the sixth whorl, over a whorl and a half later. In a specimen from 

 Lyme Regis the development is parallel with that of obtusion, var. E, until the 

 first quarter of the sixth whorl, then the sides became flattened, the pila? de- 

 pressed, and the abdomen narrowed to the area of the channel ridges. 



Sowerby's types, figured by Wright, are young specimens, and exhibit very 

 completely the fact that the young may develop the characteristics of the species 

 at a comparatively early stage, and are more highly modified as a rule than the 

 adults of obtusion in any variety. 



The sutures on the sixth volution of a specimen from Gmiind had lobes differ- 

 ing two thirds and saddles one half; and on last quarter of same, the lobes differed 

 three fourths and saddles two thirds. In an older specimen from Semur, on the 

 second quarter of the seventh whorl degeneration had already begun, the lobes 

 differing only one fifth to one sixth, and saddles about two fifths. This and the 

 fold-like character of the pila? and trigonal form showed that old age had fully 

 begun on this whorl. There are two specimens of stellare in the Museum of Com- 



