THEORY OF RADICALS \M> MORPHOLOGICAL BQCTVALENt 



clature, are epacmic, and the tertiary are what we should call acinic radicals. 

 - an excellent example ol an acraio radical in the Jura. It Btands 

 morphologically and chronologically at the centre of the affinities of the _ 

 of 1>. ind StephanoceratidsB, that i-. of the larger part • »! the oolitic 



Ammonitinm. It ia, in ii< relation to these, and to the characteristics of their 

 of development, an epacmic radical, l>ut with regard to 

 and more ancient secondary radical forms, it i< a tertiary or acmic ra 

 It ha* a Battened abdomen, very » 1 i \ * -i j_r»- n t Bides, like tlii'-i- of Siepl 



similar acmic radical forms, and a line of coarse tubercles along the Bides. 



i tier distinct from Psiloceras, it is also a perfectly discoidal form. 



I endonts oi which belong properly t" the Btepham 



ami allied groups, are al-<> discoidal forma, though the series often have involute 



is '/ 



tiary radicals in what we pro .11 the Pettoa Stock, or Spinifera, 



to the evidence * > l~ 1 1 » « - younger Btages and the characteristics of adults. 



ne row of large Bpines in adults, and whorls which are very gibbous or 



oidal in section, that is, with abd< u broader than dorsum. The whorl 



smooth, with only one row of lateral Bpines, but i- usually 

 ;ly pilnted, the piloe being Bingle on the sides and as a rule bifurcated only 

 near to the abdomen. The sutures have a more or less close resemblance 

 to tin'-.- ol /' D I /' The line of descent being broken, 



ill, in the imperfect li>t below, give Mine forms having two lines of tuber- 

 These, however, have young which, until a I Bhow only one outer 



ml tubercles, as in tin- adult, of the two species cited above. 

 tin- Lower Oolite is the tertiary radical <>! that genus, and of M 

 cephalites, Spha Morpboceras, Reineckia, Cadoceras, Quenstedio 



ihanus, and Pacbydiscus. All of these genera have Borne 

 which are either closely similar to the radical in the adult r else 



•i a nodosum-] P w athlda has a similar hi* 



ii it i* like Dactylioceras in it- m I has two lateral rows of 



in the adult. The huge coronate 

 I J '■ nit] the - 



spined forms like and Bhells like -\y- perurmaium, 



with two rows of spines, are obviously in the line "I stock forma 

 I ■mi the discoidal shells "I these groups a more o 



allied set k forms, from each of which a separate genu* 



. until we find in the Cretaceous n new beginning in // 

 : ' ii.. .iiiiI ol Acnntho- 



1' i | il>l\ II ■ H- and < 'u-tiili- 



Th< oup, with nodose keels or lines of tubercles in | 



to the Spinifera, but thi hylum connected, in 



common with such formr with the Hopl 



and their r idical is also Ii I I 



the I /' 



i in the young ol the 



