PTEROPODA. 7 
Straits and North Pacific), the British Museum and others. The following table 
will show the points of difference which I have found to be constant :— 
LIMACINA ANTARCTICA. LIMACINA HELICINA. 
1. Maximum breadth observed 5°5 mill.; fins 1. Considerably larger. Maximum breadth observed 
about 4 mill. long. 9 mill. ; fins 10 mill. long. 
2. Shell with very fine transverse striation or with 2. Striation much stronger and more distinct : 
none at all. (But see what is said above as sometimes darker than rest of shell. 
to the injuries sustained by the shells.) 
is) 
3. Umbilicus without any sort of keel or distinct . Umbilicus surrounded by a very distinct circular 
border, even in the largest specimens. raised border, which is invariably present in 
well-preserved specimens of moderate size. 
4, The colour of the animal is light, with the 4, The upper and inner whorls are marked with a 
exception of a distinct dark mass composed dark stripe following the direction of the 
of the viscera and situated chiefly in the spiral, so that the shell when seen from above 
second half of the first whorl. The inner presents an alternation of dark and light 
and upper whorls are entirely light (fig. 1b). spiral stripes. The dark stripe is not hepatic, 
but is formed by a pigmented membrane 
which appears to be continuous with the 
mantle (fig. 14). 
5. The posterior lobe of the foot is more deeply 5. 
and distinctly divided than in L. helicina. 
6. The hair-like denticulation of the teeth extends 6. The denticulation is less developed and the cusp 
to a considerable height, so that the main consequently seems more prominent. The 
cusp is less conspicuous. The base of the base of the central tooth is hollowed out 
central tooth is fairly straight, with knobs at almost into a horseshoe shape. 
the end. 
Other points, such as the shape of the operculum mentioned by Prof. Pelseneer, 
seem to me less certain. Whether the differences tabulated above are sutlicient specific 
characteristics must depend on each naturalist’s view of what constitutes a species, 
there being no accepted definition of specific difference. But as far as the collections 
which I have examined are concerned, these differences are persistent and concomitant, 
and it seems to me that when so decided a character as the presence or absence of the 
raised border round the umbilicus is accompanied by differences in size, colour and the 
teeth of the radula, the two forms are entitled to specific rank, though the divergences 
by no means show that they have originated independently, but rather support the 
idea that they are differentiations of a common ancestor or one of the other. 
VOL. III. D 
