No. 560] CAMBRIAN HOLOTHURIANS 



505 



from the center, the center representing the phylogenetic 

 starting point of organic life. In certain limited genera 

 or in certain small groups an extraordinary progressive 

 development is undoubtedly traceable through a greater 

 or lesser extent of geologic time ; but in general there is 

 a balance between the organisms in each horizon which 

 is strikingly similar to the balance between the organ- 

 isms in every other horizon and between the organisms 

 in each of the present faunal regions, so that, taking into 

 consideration the circumstances under which the animals 

 in each horizon lived, we are not able to say with any 

 degree of accuracy that, phylogenetically speaking, any 

 one fauna, zoogeographic or paleontologic, is in toto 

 more primitive than any other. 



Keeping this in mind and speaking solely as a biolo- 

 gist, I would suggest the following disposition of the 

 genera described by Dr. Walcott : 

 Holothuroidea 



Family Elpidiidae 

 Genus Laggania 

 Genus Louisella 4 

 Family Eldoniidae (near the Elpidiidae) 

 Genus Eldonia 

 Zoantharia 



Family Edwardsiidae 

 Genus Mackenzia 

 Summary. — Eldonia is a free-swimming holothurian, 

 and is most closely related to the species of the family 

 Elpidiidae. 



In body form alone does Eldonia resemble a medusa ; 

 this general resemblance may therefore safely be dis- 

 regarded as a parallelism resulting from a similar pela- 

 gic habit. 



In the general shape of the body as well as in the 

 course of the digestive tube Eldonia approaches TrocJio- 

 sphcera (and trochophore larvae) ; but the enormous dis- 



1 Both of these genera can probably be referred, with a reasonable degree 

 them, for the' present at least, unassigned. 



