NO. 1504. 



AMERICAN PARASITIC COPEPODS— WILSON. 



715 



In atlanticus the carapace is relatively larger, being- nearly six times 

 the width of the free and genital segments; here it is only three times 

 the width of the genital segment and nmch less than that compared 

 with the free segment. 



Furthermore in atlanticus the thoracic area is relatively narrow and 

 evenly rounded posteriorly, while here it is much wider and squarely 

 truncated. 



In atlanticus also the free segment is the same width as the genital 

 segment, while the terminal joint of the abdomen is several times 

 larger than the basal. In the present spe- 

 cies the free segment projects considerably 

 over the bases of the fourth legs, and the 

 abdomen joints are of the same size. 



Specitic differences may also be noted in 

 the structure of the second antenna^, the 

 second maxillipeds, the f urea, and the four 

 pairs of swimming legs, particulary the 

 fourth pair, which are small and somewhat 

 rudimentary in the present species. 



Here also these fourth legs are connected 

 across the mid-line like the three preceding- 

 pairs. Whether the same condition ob- 

 tains in atlanticus is not stated, but it fur- 

 nishes a characteristic which is quite dis- 

 tinct from other closely allied genera. 



Although there is but a single specimen 

 of this species it is well differentiated and 

 proves to be of considerable morphologi- 

 cal importance, for it helps to settle the 

 exact relation of the different carapace 

 areas in these three subfamilies, the Cal- 

 iginse, the Trebinse, and the Euryphorin^. 

 In them, two, and usually three, of the 

 anterior thorax segments are fused with 

 the head, and there has been considerable 

 discussion b}^ various authors as to the morphological significance 

 of the different portions of the carapace. A careful study of the 

 carapace of the present species, compared with that of the three 

 species of Trebius alread}^ described, and also with that of a mature 

 chalimus of Lepeophtheirus nordmannii, recently obtained by the 

 author from the United States Bureau of Fisheries, leaves little 

 chance for further doubt. It will be seen in fig. 19 that the carapace 

 of the present species is divided transversely by three grooves. The 

 anterior of these {a) separates the frontal plates from the carapace : 



Fig. 19.— The dorsal surface of a 

 male dysgamus ariommvs, show- 

 ing areas and segments. o, an- 

 TERIOR; m, MEDIAN, AND p, POSTE- 

 RIOR TRANSVERSE GROOVES. p. 1, 

 p, 2, AND p, 3, LATERAL LOBES OF 

 THE HEAD, FIRST AND SECOND 

 THORAX SEG.MENTS RESPECTIVELY. 



