No. 560] CAMBRIAN HOLOIHURIANS 



489 



Dr. Clark remarks that in Fig. 2 on plate 13 (repre- 

 senting Mackenzia costalis) "the terminal mouth sur- 

 rounded by a jointed or notched ring is distinctly shown; 

 in the specimen I was unable to make out these points 

 satisfactorily"; I can personally vouch for the presence 

 of the "notched ring"; but after the specimen was pho- 

 tographed Dr. Walcott tells me that it was subjected to 

 an acid bath in order to remove a deposit of calcite, and 

 while it was in that bath the ring seems to have disap- 

 peared. 



Dr. Clark laments that "if Eldonia is a holothurian, 

 it becomes virtually impossible to define the class except 

 in terms of the alimentary canal ; indeed, if Eldonia is a 

 holothurian, the echinoderms themselves can be defined 

 in no other terms, for Eldonia lacks every single charac- 

 ter which justifies the customary view that holothurians 

 are echinoderms." I can not agree that Eldonia lacks 

 every characteristic echinodermal character ; but even if 

 it did and we were forced to define the class Holothu- 

 roidea on the basis of the digestive tube the Holothu- 

 roidea would merely be brought into line with very many 

 of the other animal groups. I would like to see Dr. Clark 

 draw up a definition which would successfully differen- 

 tiate the Trichoptera from the Lepidoptera, or the 

 Orthoptera from the Neuroptera, or a definition which 

 would include all the members of the Diptera, but exclude 

 all other insects. The more we learn about the various 

 types of animals the more it is impressed upon us that 

 the dividing lines between them are purely arbitrary, and 

 that there is a fundamental unity covering the whole field 

 of zoology. 



Introduction. — My study of the specimens upon which 

 these new genera and species were based was entirely 

 independent of that made by Dr. Walcott, and, on account 

 of our entirely different previous training, I approached 

 the problems presented in an entirely different way. Ex- 

 cepting Eldonia, I did not examine any of the genera in 

 detail until after his paper was in press. After its publi- 



