MASTIGETHUS. 119 



This species in its structure throughout approaches Prolopadorhynchus 

 nationale (Reibisch). It differs decidedly, however, according to Reibisch's 

 account and figures, in having the neurocirri of the first three parapodia distal 

 in position, very inconspicuous, and free only at the extreme tip, which appears 

 as a postsetal process. The parapodia of the middle region are broader and less 

 acuminate. The distal piece of the compound setae is somewhat differently 

 shaped and the teeth are much finer and more numerous. It is decidedly larger 

 in size than the type of nationale and the number of somites is greater. 



There is some doubt as to the possibiHty of maintaining Prolopado- 

 rhynchus Bergstrom on the basis of the difTerence in the ventral cirri of the first 

 parapodia. From examination of the two species of Lopadorrhynchus described 

 above I find the apparent absence of neurocirri to be due to a special modifica- 

 tion in them and change in position; and I much suspect that intergradation 

 between this condition and that in which the neurocirri are normal will be found 

 to occur, such a transition, in fact, being plain in the fourth parapodia of L. 

 nans, sp. nov. The two species at present referred to Prolopadorhynchus may 

 be separated as follows. 



a. Ventral cirri with a distinct thread-like appendage at the tip; no simple setae on parapodia caudad 



of the fourth P. appendiculatum (Southern). 



aa. Ventral cirri with no such appendage; two or three simple setae on all parapodia caudad of the 

 fourth P. nationale (Reibisch). 



Mastigethus, gen. nov.^ 



Body short, more tapered caudad, depressed, the ventral surface flattened; 

 somites few. 



Eyes present. 



Tentacles four, anterior. No median tentacle or nuchal papilla. On each 

 side of the head a large branchiform nuchal organ composed of many simple 

 branches or filaments. 



Behind the nuchal organ on each side two large tentacular cirri of typical 

 cylindroconical form and a dwarf ventral one. 



Parapodia uniramous. No true ventral cirri present on any of the first 

 four pairs. 



Both simple and composite setae present of which the latter may be absent 

 from the most anterior parapodia. 



1 /udo-Ti|, a lash, and i)do%, character. 



