AET. 11 SOME BURGESS SHALE FOSSILS — HUTCHINSON 7 



specimen, while in the very simiLar unfigured laterally compressed 

 individual a comparable plate is visible lying above the specimen. 

 In the dorso-ventrally compressed specimen 56784 no such structure 

 can be made out; it hae probably not been exposed in the splitting 

 of the shale. The best interpretation to be placed on this appendage 

 is that it formed, with the abdomen, a sort of tail fan. In the 

 individual discussed below and figured on Plate 1, Fisure 3, the 

 last appendage is show-n clearW on the left-hand side, lying in such 

 a position against the abdomen. The condition of the posterior 

 appendages, here showm as broad simple plates, indicates that there 

 w^as no essential difference between the fifteenth and the other 

 appendages. Unfortunately the condition of the anterior part of 

 this specimen is not sufficiently good to allow the number and 

 structure of the anterior appendages to be made out. 



Internal structure. — Walcott mentions a "very beautiful specimen 

 shov\^ing some details of the interior." This is presumably the indi- 

 vidual figured from his photograph in Plate 1, Figure 4. The only 

 certainly determinable structures are the alimentary canal and the 

 obliquely arranged segmental musculature. 



/Sicpposed female of Opahinia. — Walcott described two specimens 

 of the same general appearance as 0. regalis, but with a reduced 

 frontal process and with filiform caudal cerci. These specimens he 

 believed to be the females of Opabinia. It is very improbable that 

 the presence of a caudal cercus would characterize the female sex in 

 this group so that in all probability these two specimens, which are 

 very poorly preserved, represent an entirely different animal. It has 

 ]3reviously been indicated that remains of eight individuals bearing 

 a frontal process exist in the collection. In addition to these are 

 two specimens which, though quite well preserved, are without any 

 such organ. The better specimen of these is figured in Plate 1, 

 Figure 3. The head ends anteriorly as a truncate lobe in front of 

 the stalked eyes. It is possible that the process lies at a different 

 level in the shale or has been broken, the general appearance of the 

 specimen however is against such a supposition, and I am inclined 

 to think that the two individuals without frontal processes represent 

 the true female of Opabinia regalis. 



Comparison with living Anostraca. — In discussing the relation- 

 ship of Opabinia with modern Anostraca the following resemblances 

 may first be noted : 



1. Absence of a carapace. 



This characterizes two groups of Branchiopoda, the Anostraca 

 and the Lipostraca. From the solitary representative of the latter 



