594 PROCEEDINGS OF THE NATIONAL MUSEUM. vot„ xxviii. 



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

 segment, where it joins the free seg-ment, is contraeted into (jnite a 

 long neck, well creased with wrinkles. 



The body 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 consitlerably short(>r than in the adult and is 

 l)lainly segmented at about the center — i. e., it is practically the same 

 ;is in the male. There is not the slightest doul)t that these are realh^ 

 young females, for the (}.^g strings can be seen forming inside the 

 genital segment, !>nd 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 plainly to the con- 

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

 mentation is usually very well concealed. 



Accordingly a large number of adults were carefully examined with 

 the result that two were found which showed a segmented a])domen 

 very plainl3\ 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 al)domen are 

 about equal, in the adult the ])asal 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 2>i'"<^>(''^"-'^- (^ee p. G(»0.| 



The National Museum collection includes the following lots of this 

 species, all o])tained from the same fish and quite constant in their spe- 

 cific characters: Two lots, numbered 6035, ol)tained l)y Mr. Rathbun 

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

 at Woods Hole; five lots, numbered W 5(», W 51,' W 5;^, W 53, W 51, 

 ol)tained by Mr. Rathbun in Vineyard Sound, in the innnediate vicinity 

 of Woods Hole; five lots, munbered W 55, W 56, W 57, W 58, W 5!>, 

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

 mens in Mr. Rathbun's collections num))er nearly 200, while the author 

 obtained almost as many. The species must therefore l)e regarded as 

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



CALIGUS PELAMYDIS Krbyer. 



Plate XIII, ti^'s. 154-l(n, Plate XIV, lig. 161a. 



Oiligus pelamydls Kroyer, 1863, p. 50, pi. iv, tig. 4, a-g. — Richiardi, 1880, p. 

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



Fcimile. — Carapace orbicular, somewhat narrowed anteriorly, the 

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

 ol" the body, and with nearly straight lateral margins (fig. 154). 



