NO. 6.] ACCOUNT OF THE SPECIES. 95 



20. Hemicalanus spinifrons, n. sp. 

 (PI. XXVIII). 



Specific Characters. ? Body exceedingly pellucid and somewhat de- 

 pressed, with the anterior division regularly oblong oval in form, gradually 

 tapering both in front and behind. Cephalic segment rather longer than all 

 the other segments of the trunk combined, and exserted in front to a long 

 spiniform prominence pointing straight anteriorly. Last pedigerous segment 

 united with the preceding one, and having the lateral corners evenly rounded. 

 Tail very short, not attaining even Ve of the length of the anterior division, 

 genital segment rather dilated, and fully as long as the other 3 combined. 

 Caudal rami about twice as long as they are broad, sublamellar, and slightly 

 incurved at the end, which is transversely truncated. Caudal setae rather 

 strong, 5 on each ramus, 4 apical and one lateral, besides a small bristle at 

 the inner corner. Superior antenna? considerably longer than the body, and 

 25-articulate, being clothed with slender bristles, some of which are rather 

 elongated. Structure of the remaining appendages nearly agreeing with that 

 in the other known species. Length of adult female 4 mm. 



Remarks. There are 4 species to which this form bears a very close 

 resemblance, viz, H. mucronatus Glaus, H. acutifrons Giesbr., H. oxy- 

 cephalus Giesbr., and H. spiniceps Giesbr. In all these species the cephalic 

 segment is exserted in front to a similar spiniform prolongation; but as they 

 are all closely allied, it is rather difficult to determine to which of them the polar 

 form bears the nearest affinity. In its external appearance it seems to agree 

 best with H. oxycephalus, to judge from the sketch given by Dr. Giesbrecht 

 of this species; but the size is rather larger, and about the same as in 

 H. spiniceps. I give below a detailed description of the specimen found. 



Description of the Female. 



The length of the body in the solitary specimen examined, which is a 

 fully grown female, is about 4 mm., measured from the tip of the frontal 

 process to the end of the caudal rami. 



The whole body is so perfectly hyaline, even in the preserved state, that 

 most of the inner organs appear with great distinctness through the inte- 



