NO. 2 MIDDLE CAMBRIAN HOLOTHURIANS AND MEDUSA 45 



that had not yet been distorted by attachment to any foreign body 

 as in fig. 2.' 



Among the echinoderms of the Middle Cambrian we have hereto- 

 fore known only the Cystidse. To it we are now able to add several 

 representatives of the more highly organized Holothurioidea. Of 

 the six families of the Holothurioidea recognized by Ray Lankester 

 (1900, p. 226), three are represented: two directly and one indi- 

 rectly. The Holothuriidse is represented by Laggania cambria and 

 Louisella pedunculata and the Synaptidae by Mackenzia co stalls. 

 The Pelagothuridge is indirectly represented by Eldonia hidwigi. 



With the thought of returning to the field and making a much 

 more thorough search for animals of this class during the field season 

 of 19 II, I will not add to these preliminary notes or attempt to draw 

 further deductions that may soon be strengthened or disproved. 

 Certain obscure remains suggest the presence of other forms of the 

 Holothurioidea that may be of essential service in working out the 

 Cambrian representatives of the class. 



Class HOLOTHURIOIDEA Siebold, 1848 



Order ACTINOPODA 



Family ELDONIIDAE, new family 



Body medusa-like, disk-shaped. Mouth and anus ventral. Water 

 vascular system radial from aboral pole. No podia. No respiratory 

 trees. No calcareous skeleton. 



Genus Eldonia, new genus, represented by one free swimming 

 species, Eldonia hidwigi, new species, of Middle Cambrian age. 



Genus ELDONIA, new genus 



Eldonia is characterized by a depressed, umbrella-shaped, radially 

 lobed medusa-like body, with a broad band of concentric muscle fibers 

 on the outer half of the subumbrella surface. Mouth ventral and 

 provided with " peltato-digitate " retractile tentacles. 



^ After the above was written, I talked with Dr. Austin H. Clark, who does 

 not agree with the greater number of zoologists that the ancestors of all 

 echinoderms were attached. He called my attentio'n to his paper " On the 

 origin of certain types of crinoid stems," in which he notes the prolonged 

 free swimming stage of the larvse of Tropiometra and that the larvae of 

 echinoderms are highly specialized and fitted for quite a different mode of 

 existence from that of the adults. (Proc. U. S. Nat. Museum, Vol. 38, 1910, 

 P- 213.) 



