6 PROCEEDINGS OF THE NATIONAL MUSEUM. vol. 59. 



Arneus had been preoccupied by Gistler for a genus of Coleoptera 

 in 1848. 



In 1865 Heller, in dealing with the Crustacea of the Novara ex- 

 pedition, 5 showed that both Kollar's museum label and Kr0yer's pub- 

 lished species were synonyms of Gerstacker's species. He gave a 

 much better and more detailed description, and illustrated it with 

 excellent figures, but did not change the original genus diagnosis. 



This was left for T. and A. Scott, who in their excellent work 6 

 gave the following genus diagnosis: 



First three thoracic segments fused with the head ; fourth segment with two 

 dorsal plates. Genital segment lobed posteriorly. 



This is the only correct diagnosis hitherto published, and it further 

 stated that all four pairs of legs were biramose; both rami of the 

 first pair two-jointed, of the second and third pairs three-jointed; 

 exopod of the fourth pair three jointed; endopod two jointed. 



Although there is only a single female, without egg strings and 

 from a practically unknown host, upon which to found the present 

 species, it substantiates T. and A. Scott's diagnosis in every par- 

 ticular, and adds to the validity of the genus by furnishing a second 

 species. 



ACHTHEINUS DENTATUS Wilson. 



Plate 3, figs. 20-27. 



Host and record of specimens. — Seven adult females were taken by 

 Dr. A. B. Ulrey from the outside surface of a thresher shark, Atopius 

 vulpes, in an aquarium at the marine station of the University of 

 Southern California, Venice, California, July 14, 1919. They have 

 been given Cat. No. 53555, U.S.N.M. Another lot consisting of six 

 adult females, two young females, and a male was obtained by J. R. 

 Beck from the outside surface of Mustelus lunulatus, 3 miles off 

 Venice, California, July 28, 1913. The male has been isolated with 

 Cat. No. 53558, U.S.N.M.; the females, old and young, have beer 

 given Cat. No. 53554, U.S.N.M. 



Specific characters of adult female. — In addition to the descrip- 

 tion given, 7 the following may be added. On the ventral surface of 

 each posterior lobe of the genital segment is a rudimentary fifth leg, 

 consisting of a large papilla tipped with two small spines. A sixth 

 segment is partially separated from the ventral surface of the genital 

 segment at about the beginning of its posterior third. It takes the 

 form of a large lobe on either side, between which and the genital 

 segment opens the vulva. The egg sacks run back dorsal to the 

 abdomen and ventral to the posterior lobes of the genital segment, 

 in a manner similar to that found in the genus Dinematura. There 



6 Reise der Novara, p. 189. 



6 British Parasite Copepoda 1913, p. 82. 



7 Proc. TJ. S. Nat. Mus., vol. 39, 1911, p. C.°.0. 



