MASTIGETHUS ERRANS. • 121 



the same form and length, reaching to the anterior border of the third setigerous 

 somite. Below the posterior of these is a minute ventral cirrus. 



The metastomial somites are much depressed. They are short and do not 

 vary much in length. Back of the second one each is double, being crossed 

 above by a distinct transverse sulcus or furrow, the anterior division being 

 shorter than the posterior in the anterior somites but equal to it or nearly so in 

 the middle and posterior regions. In the preserved specimen there is a narrow 

 neural furrow which is not evident on the first three somites. 



The first three somites are conspicuously differentiated from the others in 

 having their parapodia shorter and much more nearly cylindrical, being decidedly 

 thicker in the anteroposterior direction. They bear no true neurocirri, though 

 a thin and very narrow postsetal membrane free at the tip may be homologous. 

 The presetal membrane is very low, forming but a narrow fringe. The fourth 

 parapodium agrees with the first three in lacking a neurocirrus; but it contrasts 

 strongly in being larger and decidedly compressed in the anteroposterior direc- 

 tion, as also in having the thin presetal membrane conspicuously long, obtusely 

 angular at the middle distally, extending across entire end and projecting beyond 

 at its lateral ends. The fifth parapodium is similar to the fourth, but is some- 

 what longer and bears a typical neurocirrus. The eighth parapodium has attained 

 the maximum length. Each notocirrus is small; it is attached well toward the 

 base of the parapodium, arising from a swollen base or cirrophore; the style is 

 conical, ending in a short produced acute tip. Notocirri were not present on 

 the first two pairs of parapodia, having apparently been broken off. The 

 notocirri remain of nearly the same size throughout. The neurocirri are very 

 small and conical in shape, each arising from a distinct cirrophore a little proxi- 

 mad of the middle of the neuropodium. (Plate 18, fig. 1). 



The first three pairs of neuropodia bear only simple setae, which are stout, 

 long, distally curved, and often weakly sigmoid. They diverge widely and are 

 mostly on the ventral slope of neuropodium. The fourth neuropodia have on 

 the ventral side a series of five simple setae of which the two most proximal are 

 reduced; normally there are probably compound setae on the dorsal side, but all 

 are missing on the type, probably because of rubbing. Succeeding neuropodia 

 with but one or two (mostly two) stout, simple setae, and with numerous com- 

 pound setae both above and below. The compound setae have shafts straight, 

 or but slightly curved, gradually clavately enlarged distad, as usual, and ending 

 in an asymmetrical socket of the ordinary type, the tip of the process bent to 

 one side. The distal piece or blade is very thin and broad, of the common 



