PARASITIC COPEPODS — WILSON" 537 



narrower than the abdomen and vary in length from 1.75 mm. to 5 

 mm. 



The two segments of the abdomen are more or less fused but the 

 joint is plainly indicated by lateral invaginations. The anal segment 

 has a shallow invagination at the center of the posterior margin, the 

 corners on either side protruding in a rounded process. The caudal 

 rami are about as wide as long, are inclined toward each other and 

 rounded at their tips, each with four unequal setae on the tip and 

 outer margin. Type, 6.50 mm. long. Carapace, 1.50 mm. long, 1.35 

 mm. wide. Genital segment, 2 mm. long, 2.50 mm, wdde. Abdomen, 

 2.20 mm. long, 0.52 mm. wide. Egg strings, about 2.50 mm. long, eggs 

 rather thick. 



Rennarks. — The greatly shriveled fourth legs are such an excep- 

 tional character that every one of the specimens was examined to make 

 sure they were all alike. None were found with larger legs but two 

 or three had smaller ones and these legs thus furnish the most con- 

 venient means of identifying the species. The shriveling is all the 

 more noticeable in contrast with the size of the adult and has sug- 

 gested the specific name. Other specific characters are the unusual 

 proportions of tlie body regions and the great dissimilarity in the 

 size of the abdominal segments. 



Family PANDARIDAE 



Genus ACHTHEINUS Wilson, 1908 



ACHTHEINUS DENTATUS Wilson 



Plate 33, Figures 196-20S 



Achtheinus dentatus Wilson, 1911, p. 630, pi. 67, figs. 22-31; 1921, p. 6 (except 

 male described on p. 8) , pi. 3, figs. 20-27. 



Thirty females were taken from the outside surface of a hammer- 

 head shark {Sphytma zygaena) caught at Estero de la Luna in the 

 Gulf of California. Of these females, 18 carried a male attached in 

 sexual union and one female had two attached males. The ventral 

 surfaces of the two sexes were together and the anterior margin of 

 the male carapace was pushed as far forward as possible under the 

 fourth legs of the female. A male, U.S.N.M. No. 79701, attached to 

 a female, has been selected as the allotype. 



Male. — Carapace subovate, considerably narrowed anteriorly and 

 reentrant on the frontal margin, as wide as long, the posterior lobes 

 reaching the fourth segment. Two pairs of dorsal plates on the 

 thorax behind the carapace, much smaller than in the female and 

 confined to the lateral margins of the segments. Genital segment 

 quadrangular, without a dorsal plate but with a rudimentary fifth 



