1887.1 PROCEEDINGS OF UNITED STATES NATIONAL MUSEUiM. 559 



DSaCRIPTIONS OF NEW SPECIES OF PARASITIC COPEPODS, BE- 

 LONGING TO THE GENERA TREBIUS, PERISSOPUS, AND LBR- 

 NANTHROPUS. 



By KBCIIAUU UATIIBrN. 



(Witli Plates XXIX-XXXV.) 



Trebius tenuifurcatus Rathbuu, new species. 



Plate XXIX ; Fig.s. !-:{. 



This species dititers from TrehiuH caudatus Kroyer in Laving a propor- 

 tionally much smaller cepbalothorax, wbile the tail is apparently jointed 

 only near the middle, being three-jointed in the latter species. It is 

 founded upon two specimens, a female with egg-tubes attached and a 

 cast skin of the same sex, both of which were taken from a sting ray 

 captured in Vineyard Sound, Massachusetts, by the Fish Commission 

 in 187 L They have only recently been examined, and the single com- 

 plete specimen, the only one upon which reliance could be placed in 

 determining the shape and proportions of the segments of the body, has 

 be(;ome hardened and somewhat distorted after long ])reservation in 

 alcohol. The dorsal view represented in Pig. 1, Plate XXIX, is to be 

 considered, therefore, as only approximately correct and not at all com- 

 plete. The transparent border to the cephulothorax is mostly de- 

 stroyed, and no attempt has been made to represent it. The feet, which 

 project from the sides of the body back of the cephalothorax, have also 

 been omitted, not being in the proper condition to show their precise 

 positions. 



The cephalothorax is nearly one-third the length of the entire bodj^, 

 its margins very regularly curved, with the posterolateral corners 

 reaching about as far back as the i)osterior margin of the first free tho- 

 racic segment. Its width slightly exceeds its length and is greatest 

 posteriorly. The first free thoracic segment is wide and short, the sec- 

 ond much uarrower and longer, and rounded in outline. The exact 

 proportions and shape of the genital segment are not determinable. 

 The tail is very slender, elongate, somewhat more than oue-third the en- 

 tire length of the body, and is articulated very near the middle. 



The appendages agree for the most part very closely with those of 

 Trebiufi caudatus, but the furca presents sufficient dififereuces to afford a 

 .uood means of distinguishing the species. In caudatus, the furca (PI. 

 XXIX, Fig. 5) is broad with short and stout rami, the anterior ovate por- 

 tion being very large, only slightly horny, and separated from the furcate 

 portion, which is very thick aiul rigid. In tcnuifurcatuSj on the contrary, 

 the furca (PI. XXIX, Fig. 3) is relatively narrow and united in one piece^ 

 which is of uniform consistency throughout. The anterior portion is 



