344 P. E. Raymond— History of Corals, etc. 



thermore, a possibility that Mackenzia is even more prim- 

 itive than is suggested by Dr. Clark. Although his inter- 

 pretation of the ring about the mouth as a series of con- 

 tracted tentacles is a very probable one, still their small 

 size suggests that they are simple *^out-pushings," and 

 that the actinian of the Middle Cambrian retained in the 

 adult a structure like that of the larva of the modern 

 actinian just before fixation (fig. 2). If further discov- 

 eries sustain this opinion, then Mackenzia can be added 

 to the list of ^^ ancestors ^^ whose recognition has been 

 brought about by the aid of the doctrine of ^'recapitu- 

 lation. ' ' 



Figs. 1-3. 



Fig. 1. 

 ural size. 



Fig. 2. 

 Appelof. 



Fig. 3. 



Edwardsia heautempsi. 



TJrticina crassicornis. 

 Much enlarged. 

 Mackensia costalis. Somewhat restored. 



After de Quatrefages. Two-thirds nat- 

 Larva just before fixation. After 

 After Walcott. Two- 



thirds natural size. 



The chief interest in Mackenzia lies, however, in the 

 fact that it is a limeless Zoantharian which can be placed 

 in a primitive position in relation to modern corals. 

 Hexacoralla pass through a Mackenzia-Edwardsia stage, 

 and it is very probable that the Tetracoralla did also. 



One of the puzzles in the history of the corals has been 

 the abrupt replacement of the Tetracoralla by the Hexa- 



