966 MR. F. A. POTTS ON 
those of the other species, though much smaller. But they’ 
have six or seven teeth, or very nearly the number which is found 
in WM. taylori. 
The second segment is about two-thirds the length of the 
first. The notopodia of this segment are exactly like those 
described as typical for the genus—conical and enlarged, with a 
groove running down the internal border towards the median 
eroove. It contains a couple ot siender acicular sete, which do 
not project terminally from the parapodium as in the first 
segment and have no distinguishable head. The neuropodia are 
double structures and call for no remark. 
About the middle of this segment there is an enlargement of 
the lips of the ciliated groove rather like those occurring in the 
second and third median segments of I. taylori. Itis exceedingly 
interesting to notice, however, that in some individuals the lips 
approximate posteriorly, and an almost complete circular eup is 
formed like that in the 13th segment of Chetopterus. This is 
a variable character in the species, however—in the individual 
figured here there is no posterior fusion of the lips. But from 
the variations which occur in the genus Mesochaetopterus we can 
undoubtedly see how the accessory feeding-organ in Chetopterus 
has arisen from the ciliated groove. 
The posterior region (Pl. III. figs. 7, 8) is composed of short 
rounded segments. As none of the specimens is complete, I am 
unable to say how many are found. Anteriorly each segment 
is divided into two by a slight constriction. The proximal half 
bears the parapodium. As in J/. taylori, the free part of the 
notopodia projects very slightly from the surface. As a rule, 
they contain a couple of thin acicular sete, sometimes only a 
single one. The neuropodia have the usual double uncinigerous 
torus, the dorsal part being very small. 
The distal half of the segment is the part which in M/. taylori 
is diminished to form the neck between successive segments. 
The two species here described differ extraordinarily in size, 
for while J/. taylori is the largest Chetopterid known, JZ. minuta 
is probably the smallest. I shall have occasion to remark upon 
the great variation which occurs in the development of the 
prostomium in the genus Phyllochetupterus, but in this respect 
these two species differ still more widely. But while WZ. minuta 
resembles many species of Phyllochetopterus in the character of 
the prostomium, the peristomial collar is deeper and _ better 
developed than is ever the case in the latter genus, and the 
second pair of peristomial appendages—which are so character- 
istic of Phyllochetopterus—are, I think, absent here. These two 
circumstances are, I believe, connected. Then, again, the number. 
of segments comprised in the median-region is quite constant, 
but different in the two species—two in J/. minuta, three in 
M. taylorii—and this draws another very definite distinction. 
There are other differences, which I have mentioned above. 
Some of these, ¢. g. number and shape of sete, are partly dependent 
