18 PEOCEEDIXGS OF THE NATIONAL MUSEUM vol. 78 



are posterior. In all probability thej^ did not arise directly one 

 above the other; the axis of the appendage is doubtless somewhat 

 twisted in some of the specimens. 



Head and associated structures. — None of the hitherto unde- 

 scribed specimens show any of the head structures described by 

 Walcott. Since («) and (&) are very perfect it seems best, for 

 reasons already given, to disregard to problematic " head." The 

 actual head — that is, that part of the animal in front of the branched 

 appendages — seems to have had an irregularly truncated margin 

 which probably bore papillae. On the right-hand side of the head in 

 (a) a short appendage or frontal papilla of uncertain structure can 

 be made out. (Fig. 4 (a) jr. p.) No horny jaws can be detected, 

 though the fine preservation of the claws in this specimen makes 

 it almost certain that they would have been detectable had they 

 existed. The mouth seems to have been terminal, and a slightly 

 dilated buccal cavity is indicated. 



hiternal organs. — The unbranched alimentary canal containing 

 dark material can be seen almost throughout the whole length of the 

 body in several specimens. The position of the anus can not be 

 determined. 



Habitat. — There can be no doubt that the deposit in which Ayshe- 

 aia was fossilised is of marine origin. The evidence afforded by 

 the whole of the associated fauna points in this direction. More- 

 over, the presence of no less than nine specimens in the collection 

 indicates that the organism was not a casual straggler w^ashed in 

 from the land, but a true member of the marine association with 

 which it is found. Two morphological points are of interest in this 

 connection. 



1. The characteristic Onychophoran form of the body was appar- 

 ently as strikingly developed in the Middle Cambrian marine form 

 under consideration as in the terrestrial Peripatidae and Peripatop- 

 sidae of to-day. Many zoologists have objected to the division of 

 the living Onychophora into two families with many genera on the 

 grounds that the whole assemblage is a very uniform one in spite of 

 considerable differences in detail. In Aysheaia we have a form 

 living under entirely different ecological conditions from those of 

 the modern species, and at a very remote time, yet having an ex- 

 ternal appearance, which in life must have been extraordinarily 

 similar to that of the living representatives of the group. The con- 

 siderable structural differences which do occur are only manifest 

 when a minute examination is made. Presumabl}' the internal struc- 

 ture of the body of Aysheaia presented differences of great impor- 

 tance ; tracheae are hardly likely to have been present in a form that 

 we may reasonably assume was primitively marine. 



