THEORY OF RADICALS AND MORPHOLOGICAL EQUIVALENCE. 25 
Cal. tortile, Cal. laqueum, and Schiot. catenata, in the Plicatus Stock of the 
Arietide, are more closely allied to one another and to Psil. planorbe than are the 
morphological equivalents among their descendants to one another. However 
closely the descendent involute forms may simulate one another, their nepi- 
onic and nealogic stages are generally distinct, and indicate the series with 
its peculiar differential characters. Arn. miserabile or semicostatum, and Agas. 
levigatum, are more nearly related to each other and to Psi, planorbe 
in the Levis Stock than are any of the descendent morphological equiva- 
lents. ‘There are several forms closely representative of one another, and ap- 
parently almost identical, among these morphological equivalents. Thus the 
adults of Ver. Conybeari are apparently very closely allied to Oor. bisuicatum, 
and to some forms of As¢. Turnerd and Arn. ceras ; but all of these are more 
distinct in their nealogic stages than in the adults. The Arietidee present in 
this respect a similar picture (Summary Plates) to that of the whole group of the 
fossil Cephalods. Thus the adults of the earlier and simpler radical species, from 
which the later and more complicated forms must have been derived, are more 
closely related in structure than any of their adult descendants. The Cyrtocera- 
tites, Orthoceratites, Gyroceratites, the Nautilini, and the anarcestian Goniatites of 
the Silurian, are more nearly related in structure and development, in the simi- 
larity of the adult sutures, the absence of pil and tubercles, and the mode of 
growth, than are their direct descendants, the genera of the Nautiloidea and the 
Ammonoidea in the Carboniferous and Jura. 
The Nautiloids and the Ammonoids had morphological equivalents, but close 
parallelism is not constant or frequent, and occurred principally among later 
forms. We have elsewhere discussed this question, and need only notice well 
known cases; such as the extraordinary likeness of Clydonautilus to the higher 
forms of Goniatitine due to its divided ventral lobe, of Centroceras to Agonia- 
tites, and of Subclymenia to Agoniatites, and also the better known example 
of the Clymeninz of the Devonian and the Aturia group of the Tertiary. Such 
cases of morphological equivalence are disposed of by the use of the convenient 
expression, that these are mere analogies. This expression, however, fills noth- 
ing but a verbal gap. It neither explains parallelisms, nor the confusion they 
have occasioned and still occasion in our classifications, nor the constant ten- 
dency of straight shells to become coiled and of already coiled discoidal shells 
in progressive series to become involute, to whatever series they may belong, or 
wherever they may be found, thus producing morphological equivalents in great 
numbers. 
The only comparison that represents all these relations to my mind is that 
of a number of divergent branches united at their bases or radical ends into a 
common trunk. The branches are composed of groups, which, though distinct, 
and having differential characteristics, are nevertheless similar in the forms pro- 
duced, and in the order of procession of these forms. 
The equivalent forms of the larger branches would be admitted to have origi- 
nated independently of the direct influence of inheritance. We think that this 
is also true in the smaller series, since in no case can the similarities of the 
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