NEMERTEANS 159 



10. TETRASTEMMA NIGRIFRONS sp. nov. 



pi. xv, fig. 7; rl. xvi, figs. 6-9; pi. xvii, fig. i ; pi. xx, fig. 16; 

 pi. xxi, figs. 15-23. 



Body of moderate proportions, or rather slender for genus, rounded 

 throughout. Blood corpuscles red in color. Head more variable in 

 size and shape than in many related species ; provided with two pairs 

 of lateral, oblique furrows of more than ordinary distinctness. In pi. 

 xv, fig. 7 and pi. xvi, fig. 6, the head is represented as being well ex- 

 tended, while in pi. xvi, fig. 8, and pi. xvii, fig. i, it is considerably con- 

 tracted. The anterior pair of furrows lies between the anterior and 

 posterior pairs of ocelli ; each furrow extends on dorsal surface from 

 lateral border obliquely backward toward the median line, but fades 

 out gradually before meeting its fellow ; on the ventral side the fur- 

 rows extend obliquely forward and join in the median line not far 

 behind the rhynchodaeum opening (pi. xvi, fig. 7). The posterior 

 pair of furrows is situated just behind the posterior pair of ocelli, and 

 serves to mark off the head from the body. Conspicuous lateral con- 

 strictions at this point often accentuate the position of these furrows. 

 They, too, pass obliquely backward on dorsal surface, but are not very 

 distinctly marked in most cases. 



Color. In peculiarities of color and markings this species presents 

 more variations than any other Nemertean described in this paper. 

 A superficial examination led me to believe that no less than three 

 species were represented by individuals which on more extended 

 study were found to present all degrees of intergradation. They all 

 agree in internal organization, and are all similar in having a whitish 

 or pale yellowish head provided with a dark, dorsal marking of vari- 

 able size and shape. The under side of head is whitish or very pale 

 in all varieties. All agree, moreover, in having a deep red color in 

 the blood corpuscles. The three more distinct color varieties suggest 

 the varietal designations purpureum, bicolor andpaltidum. 



Variety purpureum (pi. xvii, fig. i). Head opaque white, with 

 large, shield-like dorsal marking of very dark brown color. This mark- 

 ing is rounded in front, and deeply bilobed in the median line ; it is 

 somewhat wider posteriorly and commonly shows two conspicuous 

 indentations near its posterior end; posterior border of marking 

 usually straight. The white color of head extends a short distance 

 back of the marking, or as far as the posterior oblique furrows, where 

 it abruptly changes to a deep rich purple, which color extends to pos- 

 terior end of body. The individual shown in pi. xvii, fig. i, is repre- 

 sented as somewhat contracted, and the white color behind the marking 



