338 PROCEEDINGS OF THE NATIONAL MUSEUM. vol. 39. 



irregular chitin rib; neck relatively wide and long; cutting blade 

 curved well forward and terminating in a tuft of long bristles ; palp 

 the same length as the neck and toothed along the posterior margin. 

 First maxillse small and weak, each knob bearing two nonplumose 

 setae, of which the outer is somewhat the larger. Basal joints of the 

 second maxillae very large and triangular in shape, reaching out on 

 the surface of the head far beyond the base of the mandibles; the 

 terminal joints relatively short and weak, their tips densely covered 

 with spines. 



First four pairs of legs biramose, all the rami three-jointed except 

 the exopod of the fourth pair, which is two-jointed; the fifth pair are 

 reduced to a mere pimple tipped with a single tiny spine. The basal 

 joints of all these legs are much narrower than is usual in this genus; 

 the following is the arrangement of the spines and setae: 



First exopod, 0-0; 0-1; II-4: endopod, 0-0; 0-0; 0-6: second 

 exopod, 0-0; 0-1; 0-7: endopod, 0-0; 0-0; 0-7: third exopod, 

 0-0; 0-1; 0-6: endopod, 0-0; 0-0; 0-6: fourth exopod, 0-0; 0-6: 

 endopod, 0-2; 0-2; 0-4. 



Color a uniform milky white in mature specimens, the more the 

 internal ovaries are developed the whiter the color. As the eggs are 

 also white until nearl}^ ready to hatch, these copepods show up in 

 strong contrast to the red gill filaments. 



Total length, 0.75 mm. Length of cephalothorax, 0.5 mm. Width 

 of same, 0.33 mm. Length of egg-strings, 0.8 mm. Width of same, 

 0.28 mm. 



(manicatus, furnished with long sleeves, in allusion to the overlap- 

 ping joints of the second antennae.) 



This tiny parasite is very common on the gills of the silversides 

 minnow, Menidia notata, along the Atlantic coast. The National 

 Museum collection includes the following lots, all obtained at or near 

 Woods Hole, Massachusetts: 38612, 38614, 38615, 38621, 38623, and 

 38626. The 25 specimens in No. 38621 are made the types of the new 

 species. Most of the drawings illustrating this species were made by 

 Dr. Richard Rathbun, assistant secretary in charge of the National 

 Museum, and were generously turned over to the author, with many 

 notes. For this efficient assistance sincere thanks are here returned. 



ERGASILUS SIEBOLDII Nordmann. 



Ergasilus sieboldii, Nordmann, 1832, p. 15, pi. 2, figs. 1 to 9.— Kroyer, 1863, 

 p. 237, pi. 13, figs. 2, a and 6.— Claus, 1875, p. 339, pi. 23, figs. 12 to 18.— 

 Gadd, 1904, p. 4, pi. 1, figs. 20 to 25. 



Nordmann's original description and figures of this species were 

 excellent, and as this was the first species he described, it would 

 naturally become the type of the genus. This position it is in every 

 way fitted to fill, since it is by far the most common of the European 

 species and is found upon the greatest number of hosts. 



