abstracts: paleontology 531 



as a parallelism resulting from a similar pelagic habit. In the general 

 shape of the body as well as in the course of the digestive tube Eldonia 

 approaches Trochosphaera (and trochophore larvae); but the enormous 

 discrepancy in size, the broad fringe about the body, the large tentacles 

 on either side of the mouth, the absence of muscles of the group type 

 characteristic of the rotifiers, and the submarginal anus, seem to nega- 

 tive the idea that the two can be in any way related. The medusoid 

 body form, the absence of a protrusible proboscis and the presence of 

 a large branched tentacle on either side of the mouth appear to offer 

 conclusive evidence that Eldonia cannot be a worm. The digestive tube 

 of Eldonia resembles that of the heteroradiate echinoderms, and espe- 

 cially that of certain holothurians; the tentacles on either side of the 

 mouth suggest an affinity with the holothurians; the radial canals 

 leading to a central ring are comparable to the radial canals and the 

 central ring of the holothurians; the broad circular muscle about the 

 body suggests a modified longitudinal holothurian muscle, and is of 

 the group type characteristic of the echinoderms; the broad brim about 

 the body is strikingly similar to the brim developed in certain elpidiid 

 holothurians, such as Euphronides tanneri and Scytoplanes typicus. A 

 pelagic holothurian is knoAvn as an inhabitant of the recent seas; tho 

 very different in origin and in affinities from Eldonia it demonstrates 

 that a pelagic habit is not impossible in the group. The species of the 

 family Elpidiidae are preeminently inhabitants of the deep sea; this 

 suggests that the fossil representatives of the family should be found in 

 very early geological formations. Therefore Eldonia is a pelagic holo- 

 thurian, related to the species of the family Elpidiidae. 



No marine animals are known outside of the holothurian family El- 

 pidiidae which have a body form like that of Louisella pedunculata in 

 all its details; but this species agrees in every particular with one or 

 other of the species in that family. We cannot, therefore, escape the 

 conclusion that Louisella peduncidata should find a place in the family 

 Elpidiidae along with all the recent animals which in any way resemble 

 it. 



By exactly the same reasoning Laggania camhria is assigned to a 

 position in the same group. 



The type specimen of Mackenzia costalis shows a pleated structure 

 which can only be interpreted as due to longitudinal mesenteries, prob- 

 ably eight in number; there appear to have been sixteen processes 

 around the mouth which probably indicate tentacles retracted before 

 preservation; the distal portion of the body resembles closely the distal 



