THEORY OF RADICALS AND MORPHOLOGICAL EQUIVALENCE. 23 
clature, are epacmic, and the tertiary are what we should call acmic radicals. 
Cel. Pettos is an excellent example of an acmic radical in the Jura. It stands 
morphologically and chronologically at the centre of the affinities of the group 
of Dactyloide and Stephanoceratide, that is, of the larger part of the odlitic 
Ammonitine. It is, in its relation to these, and to the characteristics of their 
nealogic stages of development, an epacmic radical, but with regard to Psilo- 
ceras, and more ancient secondary radical forms, it is a tertiary or acmic radical. 
It has a flattened abdomen, very divergent sides, like those of Steph. coronatum, 
and similar acmic radical forms, and a line of coarse tubercles along the sides. 
Though altogether distinct from Psiloceras, it is also a perfectly discoidal form. 
The direct descendants of Pettos, which belong properly to the stephanoceran 
and allied groups, are also discoidal forms, though the series often have involute 
species, such as Maer. macrocephulum, ete. 
Tertiary radicals in what we propose to call the Pettos Stock, or Spinifera, 
according to the evidence of the younger stages and the characteristics of adults, 
have but one row of large spines in adults, and whorls which are very gibbous or 
trapezoidal in section, that is, with abdomen broader than dorsum. The whorl 
may sometimes be smooth, with only one row of lateral spines, but is usually 
strongly pilated, the pile being single on the sides and as a rule bifurcated only 
on or near to the abdomen. ‘The sutures have a more or less close resemblance 
to those of Der. Dudressier’, or Cel. Pettos. The line of descent being broken, 
we shall, in the imperfect list below, give some forms having two lines of tuber- 
cles. These, however, have young which, until a late stage, show only one outer 
line of lateral tubercles, as in the adults of the two species cited above. Steph. 
nodosum of the Lower Odlite is the tertiary radical of that genus, and of Macro- 
cephalites, Spheroceras, Morphoceras, Reineckia, Cadoceras, Quenstedioceras, 
Aspidoceras, Olcostephanus, and Pachydiscus. All of these genera have some 
forms which are either closely similar to the radical in the adult stages, or else 
have young with a nodosum-like stage. Peloceras athieta has a similar history, 
though it is like Dactylioceras in its nealogic stages, it has two lateral rows of 
large spines, and is similar to Asp. perarmatum in the adult. The huge coronate 
forms of the Upper Jura, like Olcostephanus Gravesianus, etc., and the single- 
spined forms like Aspidoceras Hdwardsianus, and shells like Asp. perarmatum, 
Rupellense, etc., with two rows of spines, are obviously in the line of stock forms. 
In fact, one can select from the discoidal shells of these groups a more or less 
closely allied series of stock forms, from each of which a separate genus or series 
of genera arose, until we find in the Cretaceous a new beginning in Hoplites 
ftoyerianus and Cornuelianus for the species of that large genus, and of Acantho- 
ceras, Pulchellia, and possibly Holcodiscus and Costidiscus. 
The cretaceous group, with nodose keels or lines of tubercles in place of a 
keel, also belong to the Spinifera, but they form a separate phylum connected, in 
common with such forms as Acanthoceras mammillare, with the Hoplites series, 
and their radical is also Royerianus. The radical of Cosmoceras, Cos. Taylori of 
the Lias, is a species with two rows of spines allied to Deroceras armatum, and 
the adult characteristics of this species are repeated in the young stages of the 
