20 



BULLETIN 15 8, UNITED STATES NATIONAL MUSEUM 



3. 



KEY TO THE SPECIES (BOTH SEXES) 



First basipod of fifth legs with its inner margin naked, or armed 



with sliort setae or hairs tonsus (p. 20) 



First basipod of fifth legs with its inner margin armed with 



coarse spines arranged like saw teeth 2 



Posterior corners of fifth thoracic segment pointed hyperboreus (p. 21) 



Posterior corners of fifth thoracic segment rounded 3 



Head fused with first thoracic segment minor (p. 22) 



Head separated from first thoracic segment 4 



Caudal rami twice as long as wide or more; left fifth endopod 



of male three-fourths as long as exopod finmarchicus (p. 23) 



Caudal rami less than twice as long as wide ; left fifth endopod 



of male less than half as long as exopod helgolandicus (p. 25) 



CALANUS TONSUS Brady 



Figure 7 



Calamis tonsus Bkady, Voyage of H. M. S. Challenger, vol. 8, pt. 23, Copepoda, 

 p. 34, pi. 4, figs. 8-9, 1883. — Giesbeecht and Schmeil, Das Tierreich, Lief. 

 6, Copepoda, p. 19, 1898. 



Occurrence. — Five females from Georges Bank in surface tows ; 6 

 females in a vertical haul at Station 20107, GruTnpics, east of 



Cape Cod. 



Distribution. — Atlantic and Pa- 

 cific Oceans (Brady) ; northern Pa- 

 cific and Queen Charlotte Islands 

 (Willey); Gulf of Guinea (T. 

 Scott) ; Madeira and Canary Is- 

 lands (Thompson). 



Color. — Body colorless, transpar- 

 ent, and not easy to detect in ordi- 

 nary seawater. No pigment mark- 

 ings of any kind, except in one 

 specimen a faint pink spot inside 

 the body cavity above the mouth 

 (Rathbun). 



Female. — Head distinctly sepa- 

 rated from the first thoracic seg- 

 ment; first antennae as long as the 

 body and armed with small and scattered setae, except on the three 

 apical segments. Genital segment broad and swollen, one-half 

 wider than the abdomen ; inner margins of fifth basipods naked, with- 

 out spines or setae. Total length, 3.6 mm. 

 Male. — Unknown. 



Remarks. — This species is widely distributed, but nowhere abun- 

 dant ; it seems to prefer the open ocean and is not likely to be found 

 near the shore. It may be recognized by the swollen genital segment 



Figure 7. — Calanus tonsus: a. Dorsal 

 surface of female ; Jj, fifth leg 



