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. obtusion, the radical of the asteroceran series, 

 the keel was broad and low with shallow channels, and the piloa 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. 1 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 Broolci and Collenoii, 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. oxynotmn, 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 Oxyn. oxynotmn, 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 2 was more involute, more acute, and smoother even than oxynotmn. 

 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. 

 Scipionianum, and fold-like pila3. These are less pronounced in the higher 

 species, Oxyn. Gidbali and Lotharingum, in consequence of the prepotency of 

 the geratologous tendencies shown in the more compressed, more involute, 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, that in all individuals and series the 

 sutures were the last to yield to degeneration, and the characteristics of these are considered 

 by most aidhors as the pre-eminent differentials of the Arietidw. 



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 a more 

 specialized series is contrasted with an ancestral radical species or series, then 



1 Summ. PI. xiii. fig. 2. 2 Summ. PI. xiii. fig. 12. 



