210 GENESIS OF THE ARIETID.E. 



Asteroceras Brooki, Hyatt. 



Plate IX. Fig. 5-7. Summ. PI. XIII. Fig. 4. 



Amm. lirooli, Sow., Min. Conch., II. p. 203, pi. cxc. 



Amm. Brooki, Quenst., Amm. Schwab. Jura, pi. xx. fig. 11, 12; pi. xxi. fig. 1 (not pi. xv.). 



Ariel. Brooki, Whight, Lias Amm., p. 280, pi. vi. fig. 4, 5. 



Localities. — Lyme Regis, Bempflingen? 



The pilce are very close together and well defined on the first quarter of the 

 third volution. On the last quarter of the sixth volution, peculiar geniculae are 

 formed by the abrupt bending of the straight pilae, which contrast forcibly with 

 the more gradual curves of these parts in Ast. Turneri. On the first quarter of 

 the sixth whorl these are even better marked, owing to the depression of the 

 abdominal parts and greater distance of the genicular from the channel ridges. 

 There are, however, specimens in both species which do not differ in their pilae 

 at any- stage, and are precisely intermediate in all characteristics (Plate IX. 

 Fig. 5-7). The channels are perfectly well defined, and the lateral ridges are 

 entire. The channels broaden out rapidly on the latter part of the fifth whorl, 

 but do not increase perceptibly in depth, and have probably already reached 

 their full adult development. 



The differences between the young of this species and the young of Ast. Tur- 

 neri are considerable. The young of the latter throughout the fifth volution had 

 a whorl with a dorso-abdominal diameter but very little longer than the trans- 

 verse, while at a still later stage and in the adult the dorso-abdominal diameter 

 is two sevenths longer than the transverse. This last is about the proportion of 

 the same diameter upon the first quarter of the fifth whorl in the young of Ast. 

 Brooki. The sides begin to be convergent at a much earlier age in Brooki, and 

 the resemblance to the old of Turneri becomes very close. The amount of invo- 

 lution is also one half on the latter part of the fifth whorl and the early part of 

 the sixth volution, while in Turneri of the same age it is only one fourth, — about 

 the same as in the young of Brooki. 



The pilae retain their distinctness and the channels increase in depth and 

 breadth by growth, while the abdomen remains throughout much narrower than 

 in Ast. Turneri. The increase of the dorso-abdominal diameter of the whorl by 

 growth is more rapid in proportion to the transverse than in Turneri. The old 

 shell even on the first half of the eighth whorl envelops more than half of the 

 preceding whorl. This contrasts very decidedly with Turneri, which at the same 

 age is less involute, and the transverse diameter of the whorl near the dorsum is 

 but little shorter than the sides, the breadth of the abdomen being only about 

 two fifths less than that of the sides. In this species the dorsal diameter is nearly 

 a third less than the breadth of the sides, and the breadth of the abdomen is two 

 thirds less. 



The largest specimen measured had seven and a half volutions, and the diam- 

 eter was 241 mm. 



In the Museum of Stuttgardt from the Obtusus bed there is one specimen of 

 this species labelled Amm. Turneri, Sow., Pleinsbach, No. 4187. 



