252 
In the following report of the species I commence with Mitella 
polymerus as the material gives a most complete picture of this species. 
Mitella polymerus (Sowerby) Pilsbry. 
La Jolla, California, on the Coastal rocks. 21/VII 15. Forma typica , in 
great abundance. 
Bird Rock, La Jolla, California. 27/VIII 15. Forma echinata, four small 
specimens on sea weeds. 
San Pedro, California. 27/IX 15. Forma echinata ; some few specimens. 
The large material of typical Mitella polymerus displays some 
variation in the lower rows of latera. Gru vel (1905) in his diag- 
nosis gives as characteristic of the genus „Sous-rostre et sous- 
caréne toujours présents“. Nevertheless, we cannot always find a 
subrostrum in the present species; quite on the contrary, in most 
cases a subrostrum is absent, or at all events so difficult to trace 
that its presence is in faet very doubtful; the subearina, on the 
other hånd, is always well developed. 
In the material a great many very small stages are found at- 
tached to the outgrown specimens, and I was able to find the com¬ 
plete series from pupa to adult. This was indeed of great interest 
as the informations of the development of the species, which may 
be gathered from the literature, are very meagre and dissatis- 
factory. Darwin (1852, p. 310) has studied a young Mitella poly¬ 
merus of 0,oi8 inches; but in this specimen already 22 or 24 
plates were found. Nussbaum (1890) has evidently not laid any 
stress on the study of the young animals, and his drawings of 
them are indeed little precise. The deductions, which Gru vel 
(1905, p. 5) has made on basis of these drawings, therefore can¬ 
not hold against critics. 
The pupa (Fig. 14a) is very small; it is often seen crawling 
among the peduncle scales of outgrown animals, and evidently often 
fixes itself to them. As soon as it has chosen its place and fixed the 
antennae, five primordial valves appear; as in Scalpellum, and 
other cirripeds investigated, the primordial valves — the embryonic 
carina, terga, and scuta — are chitinal with no trace of carbonate 
of lime, and have the same porous structure. On the interior side 
of the primordial valves calcification now at once commences; this 
makes the primordial valve appear as a scale, indicating the umbo 
