FIFTH, OR AGASSICERAN BRANCH. 207 



parative Zoology, which are so similar to Ast. Turneri that they differ in only one 

 characteristic, namely, the shallowness of the umbilicus, due to the gradual 

 curvature of the sides. In Ast. Turneri the breadth of the dorsum, and the con- 

 sequent abruptness of the umbilical shoulders, is a marked peculiarity, even at 

 an early nealogic stage. These specimens were in the nealogic stage ; if older, 

 they would probably also have shown a less amount of involution than in Ast. 

 Turneri. This last species becomes generally, though not invariably, larger in 

 five volutions than the former in the course of five and a half. Specimens of 

 stettare also approximate very closely to Ast. acceleratum ; they differ, however, in 

 the less involution of the whorls, and in other correlative characteristics. 



Depressed tubercles similar to those of the young of obtusum appeared, in the 

 only specimen in which they could be detected, on the last quarter of the second 

 whorl, and true pilae on the third quarter of the third whorl, earlier by one 

 volution than in var. E of obtusum. Thus even in this unimportant character 

 acceleration appears to have taken place as it had in other characters. 



There is one specimen of this species in the Museum of Stuttgardt, from the 

 Geometricus bed of Goppingen, though it is more common in the Obtusus bed. 

 The largest specimen in this Museum reached a diameter of 450 mm. (PL X. 

 Fig- 1, 2). The last whorl was perfectly smooth, the abdomen had become sub- 

 acute, and the channels obsolescent, resembling those of the adults of Ast. 

 Cottenoti. 



Asteroceras acceleratum, Hyatt. 



Plate IX. Fig. 4. Plate X. Fig. 3. 



Locality. — Semur. 



The involution covers two fifths of the sides on the sixth whorl, and one half 

 of the second quarter of the eighth, whereas in Ast. stellar e the extreme limit of 

 involution is one third. The whorl is similar to that of this species, but is much 

 broader abdomino-dorsally. The umbilical shoulders are large and abrupt, the 

 dorsum much broader than the abdomen, the latter being but little wider than 

 the area of the channels. The latter are very broad and shallow, with smooth 

 but depressed lateral ridges. The keel is well marked, but depressed as in 

 obtusum. The abdomen is therefore quite different from that of either Broolci 

 or Turneri. 



The young are similar to stellare until a late nealogic stage, and differ only in 

 the greater involution of the whorls in later stages, and in the earlier develop- 

 ment of the senile folds and trigonal form. The umbilicus could be observed in 

 only one specimen. In this the pilse began with coarse folds on the last quarter 

 of the second whorl, which developed into true pilse on the first quarter of the 

 fifth whorl. 



The largest specimen from Semur measured 202 mm., and had completed the 

 seventh, and part of the eighth volution. The breadth of the first quarter of 

 the eighth whorl measured on the sides was 77 mm. There is a specimen in 

 the Museum of Stuttgardt, labelled Amm. slellaris, found at Goppingen in the 



