VJi GENESIS OF THE ARIETIDiE. 



longer than the superior laterals, and the inferior lateral saddles were one third 

 deeper than the superior laterals. 



The young are generally identified with the young of bisulcatum in France 

 and Germany. They resemble this species very closely, but have more diver- 

 gent sides, whorls much stouter, and do not have the smooth period so fully 

 shown in the young. They equally closely resemble in worn specimens the 

 untuberculated variety of Verm. Conybeari, and are often, especially in large speci- 

 mens, mistaken for Conybeari when the tubercles are either slightly developed or 

 obscured. All of these resemblances, however, are merely transient, and not of 

 the same value as the precise agreement of the younger and older stages in this 

 species and in Bucldandi ; it is in fact a more highly specialized bucklandian form, 

 with better defined channels and smaller and finer pilse. The resemblances in 

 the aspect of the young to Cor. Mum are not sufficient to unite the species. 



Canavari, in his Unterer Lias v. Spezia, 1 describes and figures Cor. (Ariel.) 

 sincmuriense, PL XX. Fig. 1, which may be the young of that species. Cor. 

 (Ariet.) Moniicillense, Fig. 3, 4, and rotiforme, Fig. 12, may be both the young of 

 the last named, or of some other allied species. Arid, multicostahis does not 

 resemble, so far as the lateral view given in Fig. 7 allows one to see, any 

 species that we know. 



FIFTH, OR AGASSICERAN BRANCH. 



The shells are discoidal only in the radical species and lower members of the 

 same genetic series; in the more specialized and higher forms of the same se- 

 ries the involution may exceed that of the shells of any series previously de- 

 scribed, except Schlotheimia. Thus, in shells in which the genicular are absent, 

 the whorl may pass inwards beyond the area of the abdomen, and embrace the 

 sides. The umbilicus, however, so far as observed, has never been entirely 

 covered in. Degeneration of the form of the whorl, i. e. flattening and conver- 

 gence of the sides, narrowing of the abdomen, and the advent of fold-like pilas, 

 may take place at an early nealogic stage in some species, and may be character- 

 istic of entire series. The keel is, however, retained even in extreme old age. 

 The keel takes on a distinct aspect in one series of this branch, Agassiceras, and 

 is transitional to the true hollow keel of Oxynoticeras. 2 



AGASSICERAS. 



The characteristics which distinguish this genus are principally due to the 

 exceedingly immature aspect of the lower forms, the shortness of the living 

 chambers, and the development in the higher species of tuberculated pilse in 

 association with highly convergent sides and angular keeled abdomens without 

 channels. The young of the radical species, At/as. Icevigatum, are remarkable for 



1 Paleontogr., XXIX. 



2 Quenstedt considers it to be a true hollow keel. See below, in the description of Agas. Scipionianum. 



