DIFFERENTIAL CHARACTERISTICS. 83 
described by Quenstedt, though we failed in getting positive evidence of any- 
thing more than a large solid keel. This, though distinct from the usual arietian 
structure of this part, had not the black layer above the siphon which dis- 
tinguishes the typical hollow keel of some species in Oxynoticeras. 
In the well known species, Ast. obfuswm, the radical of the asteroceran series, 
the keel was broad and low with shallow channels, and the pile were fold-like 
with either small tubercles or none, and the sutures in adults were like those 
of Coroniceras. The changes in course of growth from the divergent-sided to 
the convergent-sided whorl were rapid in some varieties, though in others the 
broad-abdomened and gibbous-sided whorl was retained even in adults: In Tur- 
neri, keel, deep channels, and quadragonal whorls were correlated with peculiarly 
flattened and broad sides. These species showed a tendency to specialization par- 
allel with those of Coroniceras; nevertheless, in varieties of Turneri, and in the 
succeeding forms Brook and Collenoli, the differentials, with the exception of the 
keel and sutures, tended to become extinct in consequence of the prepotent 
influence of heredity in the transmission of geratologous characters. 
Parallel phenomena were also observed, as stated above, in individuals of 
preceding series during old age, when the adult differentials disappeared, and also 
in the adult stages of certain geratologous species of the progressive Coroniceran 
series, Cor. corbiculatum, Gmuendense, and trigonatum, in which the quadragonal 
form, tubercles, etc. were similarly affected. 
In Oxyn. oxynotum, the differentials which enabled us to separate this from 
Ast. impendens and Collenoti were the hollow keel and the sutures. The hollow 
keel appeared, as has been shown, in Ozyn. oxynotum, but it was filled with layers 
of shell, though in other species it was really hollow, and appeared during the 
nealogic stages. The increase of involution was correlative with the steadily 
increasing breadth and flatness of the sides, and an intensified trigonal outline. 
Oxyn. Lymense* was more involute, more acute, and smoother even than ozynotum. 
The differentials of the Greenoughi subseries were less important characters. 
They consisted of a stouter form of whorl, which was more like that of Agas. 
Scipiomanum, and fold-like pile. These are less pronounced in the higher 
species, Oxyn. Guibali and Lotharingum, in consequence of the prepotency of 
the geratologous tendencies shown in the more compressed, more inyolute, and 
smoother whorls. 
The genera of the Levis Stock had, as a rule, shorter living chambers, usually 
less than one volution in length, and differed in this respect from the genera of 
the vermiceran branch of the Plicatus Stock. 
The important fact should be noted here, ¢hat in all individuals and series the 
sutures were the last to yield to degeneration, and the characteristics of these are considered 
by most authors as the pre-eminent differentials of the Arictide. 
In estimating certain characters as differentials, we mean only those which 
can be artificially separated and contrasted in different series of the same family, 
and which may be therefore peculiar to some one series or genus. When amore 
specialized series is contrasted with an ancestral radical species or series, then 
1 Summ. PI. xiii. fig. 2. @ Simin, Pl. xii. fis, 12. 
