192 CALIFORNIA ACADEMY OF SCIENCES. [Proc. 3D Ser. 



Dr. R. T. Jackson (1895) has been able to prove even 

 in the Paleozoic sea-urchins the possibility of correlating 

 growth stages with phylogeny, in spite of the great difficul- 

 ties due to resorption of plates, and change of form. 



Brachiopoda. — According to Beecher (1891, 1892) all 

 brachiopods go through a primitive protegulum stage, 

 correlative with the supposed ancestor of the class, although 

 Paterina, which was formerly supposed to be this radicle, 

 has been shown to be much more highly specialized than 

 the protegulum stage. The later stages of growth of this 

 class are capable of even more remarkable correlation, as 

 has been shown by Beecher (i893«) in a number of papers, 

 where every stage of growth is distinctly homologous with 

 well known pre-existing genera; and these same successive 

 genera show a gradual transition in the adults. 



Even among the Paleozoic spire-bearers ( Helicopegmata ) , 

 this holds good, for Beecher and Schuchert (1893) have 

 demonstrated that the early stages of this group are homol- 

 ogous with the terebratuloids ( Ancylobrachi'a ), and more 

 especially with the Paleozoic genus Centronella, the most 

 primitive of the loop-bearing brachiopods. 



Mollusca. — Jackson's correlations of the stages of growth 

 of pelecypods (1890) with their race history have already 

 become classic; according to these, every pelecypod begins 

 its bivalve state with a nuculoid stage, homologous with the 

 primitive radicle of the group. Every Pecten goes through 

 stages successively correlative with a nuculoid, Phombop- 

 teria, Pteruiopecten and Aviculopecten, before it reaches 

 maturity, each stage appearing in the order of the ancestral 

 genus. Even the greatly modified oyster shows its kinship 

 with this group by its nuculoid and Rhombofteria stages. 



The researches of Branco have made it clear that each 

 group of cephalopods has its typical phylembryo, in a gen- 

 eral way correlative with the radicle of the group, and that 

 the later stages may be compared very accurately with 

 ancestral families and genera. The way for this was opened 

 by Hyatt's memoirs on the ontogeny of the ammonites, in 

 which it was shown that in each perfect adult ammonite 

 shell the complete individual ontogeny is recorded. By 



