AKT. 11 SOME BUEGESS SHALE FOSSILS HUTCHINSON 9 



The frontal process is developed from the fused internal or 

 frontal appendages of the antennae in the males of various living 

 Anoslracans {Branchinella, Dendrocephalus^ ThamnocepJialus) . 

 The structure in Opahiniu though differing in detail from that found 

 in these recent forms is quite comparable in position and general 

 structure. 



These characters, most of which have been discussed very briefly 

 by previous authors, clearly indicate that Opabinia is rightly placed 

 in the Anostraca. Considerable dilTerences are, however, apparent 

 when a more detailed comparison is made. 



1. The number of body segments in Opabinia, at most IT, is less 

 than that found in any of the living Anostraca, which always have 

 at least 19. 



2. The number of appendage bearing segments, 15 in all, is on the 

 other hand greater than that found in any living forms save the 

 Polyartemiidae, and the number of postpedigerous segments, one or 

 two, is strikingly less than the 8 or 9 of all modern forms. Caiman 

 (1909) argues that the condition of the Polyartemiidae is secondary 

 because there is a postgenital abdomen of 8 segments comparable 

 to that of the other families in the group. The number of pedigerous 

 segments in this family is in fact variable, being 17 in P olyarte miella 

 and 19 in Polyartemia so that it is not at all unlikel}^ that addition 

 of segments has occurred here, as it certainly has in the Notostraca. 

 If we assume that the primitive number of segments in the Anostraca 

 is that retained by the majority of modern species, the ancestral 

 form presumably had 19 or 20 pedigerous segments. This number 

 accords well with what is known of the primitive numbers in other 

 groups of Crustacea, moreover, Lepidocaris, which is in some respects 

 the most archaic known crustacean, had 18, of which the posterior 

 one was probably compounded of two or three somites. It would 

 appear, therefore, very probable that Opabinia has a slightly reduced 

 number of body segments. The condition of the postpedigerous 

 portion suggests that, as in Lepidocaris, reduction was occurring 

 from behind. 



3. The detailed structure of the foliaceous appendage was appar- 

 ently different to what is found in living Anostraca. If branchiae 

 were really absent as seems almost certain, the appendages of 

 Opabinia show some resemblance to the anterior members of the 

 series of Lepidocar^ which also lack these structures. The modifica- 

 tion of the posterior appendage to form part of an incipient tail 

 fan is unique among the Anostraca. 



4. The absence of a caudal furca in Opahinia may perhaps be. 

 correlated with the lateral position of the posterior appendages. It 

 is not impossible, as has been indicated above, that the whole post 



26G1— 30 2 



