210 GENESIS OF THE ARIETID. 
Asteroceras Brooki, Hyarr. 
Plate IX. Fig. 5-7. Summ. Pl. XIII, Fig. 4. 
Amm. Brooki, Sow., Min. Conch., II. p. 203, pl. exc. 
Amm. Brooki, Quenst., Amm. Schwib. Jura, pl. xx. fig. 11, 12; pl. xxi. fig. 1 (not pl. xv.). 
Ariet. Brooki, Wricut, Lias Amm., p. 280, pl. vi. fig. 4, 5. 
Localities. — Lyme Regis, Bempflingen ? 
The pilz are very close together and well defined on the first quarter of the 
third volution. On the last quarter of the sixth volution, peculiar genicule are 
formed by the abrupt bending of the straight pila, 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 genicule from the channel ridges. 
There are, however, specimens in both species which do not differ in their pile 
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 As¢. 
Brooki. The sides begin to be convergent at a much earlier age in Brook, 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 7Zurneri of the same age it is only one fourth, — about 
the same as in the young of Brook. 
The pils retain their distinctness and the channels increase in depth and 
breadth by growth, while the abdomen remains throughout much narrower than 
in Ast. Turners. 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 Zurnert, 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 guar and the diam- 
eter was 241 mm. 
In the Museum of Stuttgardt from the Obtusus bed theres is one specimen of 
this species labelled Amm. Turnert, Sow., Pleinsbach, No. 4187. 
