no. 1404. PARASITIC COPEPODS—CALIGID.&- WILSON. 623 



size and shape of the carapace and of the genital segment, and he was 

 working entirely with preserved material. 



The carapace is the one part of all others to be modified in a preserv- 

 ing fluid, while a candid examination of the genital segment of any 

 common species will show that it is capable of much variation, even 

 in living specimens. We are compelled to the conclusion, therefore, 

 that we have here only a single species subject to local variations, and 

 that the various forms given as distinct species are nothing more than 

 varieties. And of them all Kroyer's gracilescens is the most worthy 

 of separation as a well marked variety. 



With reference to Steenstrup and Lutken's species hranchialis the 

 figure which they give entire shows a genital segment quite different 

 in shape from that which has been taken as typical of thomjpsoni. But 

 the detail of the appendages, of which they present most admirable 

 figures, corresponds exactly with thompsoni. And they add a partial 

 figure showing the free and genital segments and the abdomen, which 

 they declare illustrates a common variation, and which corresponds 

 very closel} 7 with that here given. Furthermore, the}^ as well as 

 Claus, give Caligus gracilis van Beneden as a synonym under Caligus 

 hranchialis. The priority of hranchialis presumably rests upon the 

 fact that it is a manuscript name given by Malm, to whom they ascribe 

 the species. But as Malm's manuscript was never published, and 

 hence Beneden's name., gracilis, antedates it by ten years, hranchialis 

 would have to be regarded as a synonym of gracilis rather than vice 

 versa. 



The important fact for us is that the two species are identical, and 

 that the one (hranchialis) for which we have reliable details seems to be 

 identical also with thompsoni. 



Finally the seven species here discussed agree remarkably in size, 

 the range being from 7 mm. in gracilescens to 8.4 mm. in thompsoni, or 

 less than a millimeter and a half, which is no more than would be 

 expected in so large a species. 



LEPEOPHTHEIRUS NORDMANNII Milne-Edwards. 



Plate XIX. 



Caligus nordmannii Milne-P^iward*, 1836-1849, pi. lxxvii, fig. 1; 1840, p. 455. 

 Lepeophtheirus nordmannii Baird, 1850, p. 275, pi. xxxm, fig. 1. — Heller, 1865, 



l». iso, pi. xvi, fi-js. 1 and 2.— Ratiihun, 1884, p. 487.— Thompson, 1895, p. 



118, pi. xxvii, fig. 5, a.— T. Scott, 1900, p. 151, pi. v, figs. 32-37. 



Female. — Carapace orbicular, as wide as long, and very- squarely 

 truncated posteriorly, or even slightly emarginate. Frontal plates 

 small and narrow and not very well defined; posterior sinuses broad, 

 deep, and well rounded. Median lobe about one-half wider than the 

 lateral lobes, squarely cut or emarginate posteriorly, with sharp 

 corners terminating in short spines. 



