PTEROPODA. 



7 



Straits and North Pacific), the British Museum and others. The following table 

 will show the points of difierence which I have found to be constant : — 



LiMACINA ANTARCTICA. 



1. Maximum breadth observed 5*5 mill.; fins 



about 4 mill. long. 



2. Shell with very fine transverse striation or with 



none at all. (But see what is said above as 

 to the injuries sustained by the shells.) 



3. Umbilicus without any sort of keel or distinct 



border, even in the largest specimens. 



The colour of the animal is light, with the 

 exception of a distinct dark mass composed 

 of the viscera and situated chiefly in the 

 second half of the first whorl. The inner 

 and upper whorls are entirely light (fig. lb). 



5. The posterior lobe of the foot is more deeply 



and distinctly divided than in L. helicina. 



6. The hair-like denticulation of the teeth extends 



to a considerable height, so that the main 

 cusp is less conspicuous. The base of the 

 central tooth is fairly straight, with knobs at 

 the end. 



LiMAOINA HELICINA. 



1. Considerably larger. Maximum breadth observed 



9 mill. ; fins 10 mill. long. 



2. Striation much stronger and more distinct : 



sometimes darker than rest of shell. 



3. Umbilicus surrounded by a very distinct circular 



raised border, which is invariably present in 

 well-preserved specimens of moderate size. 



4. The upper and inner whorls are marked with a 



dark stripe following the direction of the 

 spiral, so that the shell when seen from above 

 presents an alternation of dark and light 

 spiral stripes. The dark stripe is not hepatic, 

 but is formed by a pigmented membrane 

 which appears to be continuous with the 

 mantle (fig. 1a). 



6. The denticulation is less developed and the cusp 

 consequently seems more prominent. The 

 base of the central tooth is hollowed out 

 almost into a horseshoe shape. 



Other points, such as the shape of the operculum mentioned by Prof Pelseneer, 

 seem to me less certain. Whether the differences tabulated above are suificient specific 

 characteristics must depend on each naturalist's view of what constitutes a species, 

 there being no accepted definition of specific difi'erence. But as far as the collections 

 which I have examined are concerned, these difi"erences 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 difierences 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 dilFerentiations of a common ancestor or one of the other. 



VOL. III. 



D 



