200 GENESIS OF THE ARIETID-ffi. 



this species. His specimen as figured is, however, younger, and consequently 

 less involute, the pilue are without tubercles, and the general aspect similar. His 

 shell was, according to his figure, notably more involute than Scipionianus, and 

 this agrees also with a specimen in the Museum of Comparative Zoology. This 

 last is larger than that figured by Quenstedt, and, though comparatively senile 

 and quite smooth on the outer whorl, is more involute than any specimen of 

 Scipionianus and has a more compressed and more acute whorl. My notes taken 

 at the Museum at Semur say, " the Scipionis of Reynes is only a less involute form 

 of Scipionianus," but it is probable that the specimen observed at Semur may have 

 been exceptional, and either extremely old or a pathological example. 



Geyer, in his Liass. Ceph. Hierlatz b. Hallstadt, mentions Cymbites ghlosns, 

 Plate III. Fig. 26, a young form of Agas. Icevigatum, as cited above, and also an 

 JEriet. indet., Plate III. Fig. 13, which, if not a specimen of Agas. Scipionianum, is a 

 very close ally. These are small shells, as is usual in that locality. 



The JEgoc. Cocchi, Canav., Unter. Lias v. Spezia, Plate XIX. Fig. 11, may be 

 a young specimen of Scipionianum, but we cannot venture to refer it to this 

 genus. 



ASTEROCERAS. 



The form of the whorl is noticeably less discoidal than in the preceding series, 

 except in Asl. obtusum. The abdomen in adults is usually narrower, the sides are 

 flatter and more convergent, as well as broader, than is usual in Coroniceras. The 

 involution is not limited to the abdomen, and the whorls tend to grow inwards, 

 covering more or less of the sides. The pilae are fold-like, bending forward on 

 to the abdomen, and in most species smooth and without genicular. The young 

 have very stout gibbous whorls with divergent sides, which become more and 

 more convergent in the nealog-ic stages, and remain so throughout life. The keel 

 is constant in all the }'oung and adult specimens, but the channels are often very 

 shallow, and sometimes absent. 



The sutures have a deep abdominal lobe, but the lateral lobes are apt to be 

 short and pointed, and the saddles broad. They are similar to those of Coroni- 

 ceras, but the marginal lobes are rarely so long as in that genus. In old age the 

 abdominal lobe and the lateral lobes and saddles are of about the same length, 

 but become broader proportionally, the inferior lateral saddles become shallower, 

 and this occurs also in many dwarf forms, which of course are prematurely 

 degraded. These senile changes are more marked than in Coroniceras, though 

 the old age is not otherwise noticeably different. The genus with relation to 

 other genera of the Arietida? is notably geratologous, or in other words the shells 

 exhibit characteristics similar to those of the senile stages of Coroniceras, and these 

 characteristics may be reproduced in the adult, and even in the nealogic stages. 

 Each species and individual, however, also has a senile stage in which decline is 

 manifested unmistakably, and during this stage the whorl becomes still more 



