ON THE OCCURRENCE OF AN APUS 317 



mens came, Professor Beede writes me under date of December 14, 

 192 1, that it might be brackish water or, possibly, even fresh. 



Leaving out the A pus duhius Prestwich, described from the 

 Coal Measures of England, which, according to Beecher,^ "seems 

 to be an abdominal segment or plate of some eurypterid," there is 

 evidence of a still older form, at least closely related to Apus, even 

 in the Lower Cambrian. This is the well-known Protocaris marshi 

 Walcott from the Waucoban (Georgian) of Georgia, Vermont. Its 

 similarity to Apus was recognized by Walcott^ and has since been 

 commented upon by Clarke^ and Bernard,'* the latter author even 

 proposing to call the form Apus marshii.^ This Cambrian relative 

 has also been found in but a single example and therefore was hardly 

 a common marine form, as the tribolites collected in the same beds. 

 Therefore, even this might have drifted in from the fresh water. 



As to the remaining fossils that by some have been compared 

 with Apodidae, we refer to Pompeckj's excellent summary in the 

 chapter "Crustacea" in the Handworterhuch der Naturwissen- 

 schaften.^ The remarkable crustacean-fauna discovered by Wal- 

 cott in Burgess-Pass contains both Notostraca (Burgessia and 

 Naraoia) and Anostraca (as Opabinia, Leancholia, Yohoia), but 

 not any forms that are directly referable to Apus, though some 

 may, according to Pompeckj, be ancestral to later apodids, as 

 Naraoia to the later Carboniferous Dipeltis, which behind the 

 parabolic carapace possesses two large thoracic segments or rather 

 shields.^ The position of certain finds is considered doubtful, 

 as that of a carapace, similar to Apus, described as Lynceites ornatus 

 Goldberg from the Carboniferous of Saarbriicken and now currently 

 referred to the Cladocera. There are further to be mentioned the 

 problematic, laterally compressed carapaces of Ribeiria Sharpe 

 and Ribeirella Schubert and Waagen, which occur in the Ordovician 

 and Silurian of Bohemia, Portugal, England, and North America, and 

 which were placed by Schubert and Waagen with the Apodidae 



^ See Schuchert, 1897, p. 675. 3 1893, p. 799. 



=> 1884, p. 50 4 1894, p. 413. 



5 Schuchert (1897, p. 674), however, points to the subquadrangular shield and 

 frontal emargination as distinguishing characters. 



^ Pompeckj, 191 2, p. 789. ' Schuchert, op. cit. 



