14, NEMERTINES OF PLYMOUTH SOUND. 
Several varieties have been met with, agreeing with those de- 
scribed by other writers, but all were marbled. ‘This species agrees 
with the two following, and differs from all other Tetrastemmas in 
the rounded shape of the body, and in the absence of a well-defined 
head region, so characteristic of the remaining species. 
The breeding season is the autumn. 
13. T. NiaRUM, n. sp. 
I describe under this name a species which I at first considered 
as a variety of the last. Like Tetrastemma dorsale it has a rounded 
body, and there is no well-marked head ; but though highly variable 
in marking, it yet maintains certain characteristics to which no 
varieties of JT. dorsale that I have met with in any way approach. 
The general ground colour, which is a pale yellow, may be entirely 
or partially hidden by a strong development of a very dark brown, 
almost black pigment, which is in the form of a close network. 
This dark pigment either covers the dorsal surface of the animal 
entirely, rendering it quite black to the naked eye, or the median 
dorsal line is left uncovered, which thus appears as a median 
yellow stripe. This stripe passes from the anterior to the posterior 
extremity, and is generally interrupted, but quite irregularly, by 
bridges of the dark pigment stretching across it. The breadth of 
the stripe is very variable, both in different animals and in different 
regions of the same animal, and its edges are much frayed. The 
cephalic grooves are more apparent than in the case of 1’. dorsale, 
owing to the absence of the dark pigment at their edges; but the 
eyes, which are reddish, have the same position, and are deeply 
placed. The stylet region of the proboscis resembles that of 
T. dorsale, but there is considerable variability in the shape of the 
stylet handle. 
This species is most commonly found on Codiwm, which it im 
some degree resembles in colour, enough at least to make it difficult 
to find. I have, however, obtained it from weeds of various kinds, in 
the Laminarian zone. ‘The average length is 1 cm. 
14, T. ImMUTABILE, n. Sp. 
The specimens brought together under this name are perhaps 
identical with one described by Joubin as a variety of TJ. dorsale. 
From this species, however, they differ at least as much as many 
species (which are regarded by every one as distinct) do from each 
other. [I am, moreover, inclined to regard them as constituting a 
distinct species, because in spite of the variability of 7’. dorsale I 
have met with no variations which in any way approach them in 
