PROCEEDINGS OF THE NATIONAL MUSEUM 



TOL. 78 



In specimen 57683 there is a small crack in the exo-skeleton just 

 above the insertion of the frontal process. This crack was probably 

 formed by the flattening of some projecting structure; traces of a 

 small protuberance can be made out in the same position in the 

 unfigured laterally compressed specimen. It is possible that these 

 remains represent the point of insertion of small antennules. Alter- 

 natively these appendages may have been attached to a papilla situ- 

 ated behind the crack just mentioned and below the eye on specimen 

 57863. It is, however, quite possible that the antennules, always 

 small in the Anostraca, have become entirely obsolete. The whole 



Figure 1. — Opabinia regalis Walcott. Anterior end of cotypb (U.S.N.M. 57683) ; 



ant., ANTENNA. (ABOUT X 2.5) 



of the ventral posterior part of the head in specimen 57683 is prob- 

 ably the antenna. (Fig. 1.) This region bears a very close resem- 

 blance to the folded antenna of the female of any recent species of 

 Anostraca, and is separated from the dorsal part of the head by a 

 faint but definite groove. The frontal process, if homologous with 

 that of living forms, was developed from fused internal branches 

 of the antennae, but the latter show no other indication of having 

 been modified as extensive secondary sexual organs. No ventral 

 views of the head exist so the mouth parts remain unknown. 



Ti'unk. — According to Walcott the trunk consists of 16 somites 

 bearing foliaceous appendages. The sutures in his photograph of 



