594 PROCEEDINGS OF THE NATIONAL MUSEUM. vol. jtxvm. 



ordinarily be taken for males. The anterior portion of the genital 

 segment, where it joins the free segment, is contracted into quite a 

 long neck, well creased with wrinkles. 



The bodj^ of the segment is not much wider than the abdomen and 

 is almost squarely truncated posteriorly without any lobes at the cor- 

 ners. The abdomen is considerably shorter than in the adult and is 

 plainly segmented at about the center — i. e., it is practically the same 

 as in the male. There is not the slightest doubt that these are really 

 young females, for the egg strings can be seen forming inside the 

 genital segment, and the second antenna? and second maxillipeds have 

 the structure of those in the female and differ much from those in the 

 male. 



The examination of these developing females led plainty to the con- 

 clusion that the abdomen is segmented in the adult, although the seg- 

 mentation is usualty very well concealed. 



Accordingly a large number of adults were carefully examined with 

 the result that two were found which showed a segmented abdomen 

 very plainly. As development proceeds the genital segment widens 

 and lengthens, sending out large lobes from the posterior corners; at 

 the same time the abdomen lengthens, the increase taking place almost 

 wholly in the basal segment. 



Thus while in the immature female the segments of the abdomen are 

 about equal, in the adult the basal segment is three or four times the 

 length of the terminal. 



This development also suggests an explanation for what have been 

 considered as variations in the species productus. (See p. 600.) 



The National Museum collection includes the following lots of this 

 species, all obtained from the same fish and quite constant in their spe- 

 cific characters: Two lots, numbered 6035, obtained by Mr. Rathbun 

 at Woods Hole; one lot, numbered 1307, taken by Vinal N. Edwards 

 at Woods Hole; five lots, numbered W 50, W 51, W 52, W 53, W 51, 

 obtained by Mr. Rathbun in Vineyard Sound, in the immediate vicinit} r 

 of Woods Hole; five lots, numbered W 55, W 56, W 57, W 58, W 59, 

 obtained by the author from Woods Hole and vicinity. The speci- 

 mens in Mr. Rathbun's collections number nearly 200, while the author 

 obtained almost as many. The species must therefore be regarded as 

 a very common one on the single fish which it frequents. 



CALIGUS PELAMYDIS Kroyer. 



Plate XIII, figs. 154-161, Plate XIV, fig. 161a. 



Caligus pelamydis Kroyer, 1863, p. 50, pi. iv, fig. 4, a-g. — Richiardi, 1880, p. 

 148.— Carus, 1885, p. 357.— Bassett-Smith, 1899, p. 452.— Brian, 1899, p. 2. 



Female. — Carapace orbicular, somewhat narrowed anteriorly, the 

 same width and length, which is much less than half the entire length 

 of the body, and with nearly straight lateral margins (fig. 151). 



