BIOLOGICAL PALAEONTOLOGY 109 



be claimed as neanic expressions of morphological 

 exuberance.) Before the Liassic period ended, the 

 Cidaridae were accompanied by representatives of at 

 least two important families of Diademoida (Centrechi- 

 noida), and by the precursors of two distinct orders of 

 Irregular Echinoids. Although many new types of Sea- 

 Urchins have arisen since the Lower Jurassic, there is 

 no evidence of later, almost synchronous, outbreaks of 

 specialization comparable with that of the early Mesozoic. 

 Among the Brachiopods three of the four orders were 

 certainly differentiated in the Cambrian, and the fourth 

 is well represented in the Ordovician, possibly passing 

 back to the preceding period. Of the fourteen super- 

 families into which the four orders are divided, seven 

 date from the Cambrian, six from the Ordovician, and 

 one only from the Devonian. All known families of the 

 phylum were in existence in Palaeozoic times, thirty-four 

 appearing in the Lower Palaeozoic and eight in the 

 Upper. Similarly the Ammonoids were differentiated 

 into two suborders before the close of the Devonian 

 period. One of these, the Intrasiphonata, proved 

 unsuccessful, but no later specialization in the more 

 efficient Extrasiphonata produced modifications of sub- 

 ordinal importance. 



A phylogenetic table of any group of organisms, 

 particularly of the larger sections, resembles a bush in 

 the abundance of branches arising from the root-stock. 

 It could not rightly be called a " genealogical tree." 



Following after the neanic differentiation of orders and 

 families comes a phase of relative stagnation. Those 

 early modifications that proved unsatisfactory drop out 

 one by one, while successful developments are slowly 

 improved and stereotyped. The adolescent phylum 

 consists of a series of variously divergent lines rising 

 to the acme. Each line may have its own rate of 



