THEORY OF RADICALS \M» MORPHOLOGICAL EQUIVALENT 



' ' - in the Plicatus Stock of the 



ArietidsB, are more closely allied i<> one another and to PmL platwrb* than arc the 

 morphological equivalent* among their descendant* to one another. However 

 closelv the descendent involute forms may simulate one another, their naepi- 

 ind nealo{ - are generally distinct, and indicate the series with 



culiar differentia] characters I oi i/wct, and 



more nearly related to each other and to Pal. / 

 in the Lsevis Stork than are any of the descendent morphological equiva- 

 There are several forms closely representative of one another, and ap- 

 tly almost identical, among these morphological equivalents, Thus the 

 adults of Vtr < are apparently very closely allied to Cor. 



and to some forms of / nd A at; bul all of these are more 



distinct iii their nea jes than in the adults. The Arietidaa present in 



this respect a similar picture (Summary Plates | to that of the « hole group <>l' the 

 I ephalods. Thus the adults of the earlier and simpler radical specie.-, from 

 which the later and more complicated forms must have been derived, are more 



. related in Structure than an\ of their adult descendant-;. The Cyrtoce ra- 

 the Nautilini, and the anarcestian Goniatites of 

 S lurian, are more nearly related in structure and development, in the simi- 

 larity of the adult suture-, the absent I pike and tubercles, and the mode of 



growth, than are their din dants, the genera of the Nautiloidea and the 



Ammonoidea in the Carboniferous and Jura. 



The Nautiloids and the Ammonoids had morphological equivalents, but close 

 parallelism i- not constant or frequent, and occurred principally among later 

 form-. We have elsewhere discussed this question, and need only notice well 

 known ich as the extraordinary likeness of Clydonautilus to the higher 



GoniatitinoB due to it- divided ventral lobe, of Cent ^gonia- 



Subclymenia t<> Agoniatites, and also the better known example 

 Clymeninsa of the Devonian and the Aturia group of the Tertiary. Such 

 of morphological equivalence are disposed of by the use of the com. 



. that these are men I hi- expression, however, till- noth- 



I ther explain- parallelisms, nor the confusion they 



oil' d and Btill in our classifications, nor the con-taut ten- 



it -hell- to become coiled and of already coiled discoidal -hells 

 in pro .one involute, to whatever series they maj belo 



ver they may he found, thus producing morphological equivalents in 

 mini 1 " 



I he. .nl\ comparison that represents all these relation- t<> my mind i- that 



Of A number of divergent branches united at their bases or radical end- into a 



common trunk. The branches are composed of groups, which, though distinct, 



and having differential cl re nevertheless similar in the tonus | .1 <»- 



! in tin- order of prcweHsion of the-.- forms. 



The equivalent forms of the larger branches would be admitted to 1. 



nated independently of the direct influence of inheritance. We think that this 



• in the smalli •■! the 



1 



