No. 560] CAMBRIAN HOLOTHUBIANS 



501 



arisen from a form like Scytoplanes typicus by a further 

 sudden suppression of half of the body and the dropping 

 out of three of the rays, the rest of the body curving 

 about so as to form again a circular animal from one 

 originally pentamerous, though ultimately derived from 

 bilateral ancestors. 



Louisella. — No marine animal is known except among 

 the Elpidiidae with a body form resembling that of 

 Louisella pedunculata. A comparison between this fossil 

 and such recent species as Scotoplanes insignis shows a 

 similarity that can not but be more than superficial. 



Dr. Clark in speaking of Louisella says that "none of 

 the podia are sufficiently defined to enable one to make 

 out even the form, let alone the structure, whereas if they 

 were really like those of Scotoplanes and other elasipods, 

 their rigidity would have caused them to be as well de- 

 fined as any part of the body outline.' 1 Every one who 

 has collected specimens of certain of the species of Elpi- 

 diidae knows that they are as delicate and as difficult to 

 preserve as are many, if not most, medusae; even when 

 hardened in alcohol the podia of such forms as Deima 

 pacificum are extremely soft and flabby. If any species 

 of the group adopted a pelagic habit this character would 

 naturally be greatly accentuated. 



Laggania. — It is difficult to see how Laggania cambria 

 can be interpreted otherwise than as a holothurian of the 

 elpidiid type, a form related to such species as Bentho- 

 dytes sanguinolenta, or especially to B. sibogce. 



Circumstantial Evidence Suggesting the Possible Oc- 

 currence of the ElpidiidcE in the Paleozoic. — As I under- 

 stand the three holothurians from the Middle Cambrian 

 described by Dr. Walcott {Eldonia, Laggania and Loui- 

 sella), they all fall within, or are closely related to, the 

 family Elpidiidae. Now the Elpidiidae are preeminently 

 creatures of the deep sea, and represent possibly the 

 most strictly abyssal group to be found among marine 

 organisms. The Siboga dredged one species in 31 fath- 

 oms, and the first species to be described was found in 



