120 THE ANNELIDA POLYCHAETA. 



Proboscis with margin smooth. Bearing on each side toward the distal end 

 a single, long, isolated papilla or filament. 



Genotype. — M. errans, sp. nov. 



This genus differs from Lopadorrhynchus in having the conspicuous, long, 

 finger-like papilla on each side of the proboscis and in the presence of the highly 

 peculiar nuchal organs. 



Mastigethus errans, sp. nov.'^ 

 Plate 17, fig. 8, 9; Plate 18, fig. 1-6. 



General color of body brown, lighter at ends. Appendages in posterior 

 region verging toward brownish yellow. 



The length of the type is 7.5 mm.; the greatest width, exclusive of the para- 

 podia, 1.6 mm., and inclusive of parapodia, 3.5 mm. The body proper narrows 

 a Uttle from the anterior end over a few somites caudad and then again gradu- 

 ally widens to the middle region, from where the narrowing to the caudal end is 

 continuous and more pronounced. The total number of somites is twenty-three. 



The proboscis is shaped like an inverted, deep, square-ended scoop, being very 

 short on the ventral side but long on the dorsal; the margin is wholly smooth, 

 as is also the surface excepting for a long, slenderly cylindrical or thread-like 

 papilla on each side attached a little caudad of the slanting margin. The 

 tentacles of each side are very widely separated from those of the other. The 

 dorsal tentacle on each side is decidedly larger than the ventral one and in the 

 type projects directly laterad, curving backward at the tip. The eyes are small 

 and widely separated, one being situated immediately caudad of each dorsal 

 tentacle. The prostomium as a whole is transversely narrowly oblong, both 

 anterior and posterior margins being nearly straight. (Plate 17, fig. 8, 9). 



The branchiform organ on each side, arising in the position usually occupied 

 by the ordinary nuchal papilla, expands as a membrane from a rather narrow 

 pedicel, the membrane bearing along both the dorsal and the ventral edge a 

 series of four or five finger-like filaments, each of which is simple and is traversed 

 by a central blood-vessel. 



Peristomial rings narrow. Anterior margin bordering the mouth straight 

 and parallel with the caudal margin, which is not at all extended caudad over 

 the succeeding somite. The two principal tentacular cirri on each side are of 



' errare, to wander. 



