196 Prof. M‘Intosh’s Notes from the 
encrusting growths, so that its rounded and dilated aperture 
(trumpet-like in some) is free. From Miss Pixell’s deserip- 
tion, the tubes follow another mode of growth under different 
circumstances, 
A single example of the last species, viz., Spirorbis pusil- 
loides, De St. Joseph, kindly sent by Mr. Southern, has 
alone been under examination, and the collar-bristles in this 
instance had no gap above he, web, and might pass for those 
of 8. spirillum. Mr. Southern, however, states that he has 
seen all intermediate forms between this and the typical 
form with the differentiation at the base of the blade. 
Caullery and Mesnil also found a simple geniculate con- 
dition of the collar-bristles in preparations forwarded by 
by Baron De St. Joseph. Whether the violet pigment on 
the gut of S. pagenstecheri and the red on S. pusilloides 
denotes more than variation is uncertain. The species was 
first described by De St. Joseph under the name of Mera 
pusilla*, but Miss Bush +, seeing that the specific name was 
used by Rathke in 1836 for a species from the Black Sea, 
proposed the present title. It chiefly occurs on shells of 
oysters, though Southern found it on mussels and Trochi. 
Two forms, familiar in the literature of the subject, are 
not entered here, viz., Spirorbis heterostrophus, Montagu, 
and Spirorbis carinatus, Montagu, since both are probably 
included in the forms dealt with, and, at any rate, the 
uncertainty can only be removed by a careful examination 
of fresh animals. Miss Bush and others do not seem to 
have formed a definite opinien about either. 
Montagu (1803) { describes Spirorbis heterostrophus as 
having ‘‘a strong, spiral shell, of a dirty white colour, 
with two or three reverse velutions placed laterally, furnished 
with three longitudinal ridges, one along the back, and 
another on each side, roughly wrinkled transversely ; the 
base is flat and somewhat spreading ; aperture orbicular, 
and invariably placed opposite the sun’s apparent motion. 
Diameter not a line.” 
He procured it on oyster and other shells and on alge. It 
is readily distinguished from S. spirorbis by the longitudinal 
ridges, and by the contrary turn of the volutions. Under 
slate-stones at Kingsbridge Bay in great abundance (covering 
* Ann. Se. Nat. 7° sér. xvii. p. 35], pl. xiii. figs. 3888-392. 
+ ‘ Tubicolous a siiotids from the Pacific’ (Harriman), Alaska Exped., 
1908, p. 250. 
t Test. Brit. vol. 11. p. 50?. 
