30 
mens came from Monterey; it was afterward found at Catalina 
Island, and Mr. Orcutt now sends it from San Diego and Todos 
Santos bay; some specimens from the last locality reach 7.5 mm 
in length and 6 in greatest breadth. 
While examining some dry specimens sent by Mr. Henry 
Hemphill from San Diego some years since a very interesting 
feature was discovered which may be briefly described as follows: 
Milneria is dicecious like most lamellibranchs, and there is 
quite a difference in average size and proportional breadth, the 
male shell being always a little smaller and narrower than a 
female specimen of the same length. Both attach themselves to 
surrounding objects by a small byssus, for the passage of which 
a very slight gape exists between the ventral edges of the valves. 
The ventral surface when the valves are closed is nearly flat, an 
arrangement which has been brought about by the needs of the 
creature settling like some Arcas on a plane surface lire a flat 
stone or Haliotis back. The male has the base or ventral surface 
a little striated. In the female, however, a much more elaborate 
arrangement is found. We have in fact a proper marsupium. 
The center of the base behind the byssal fissure is pushed upward 
into a little dome nearly equally participated in by each valve. 
The edges of the valves in the arch of the dome do not quite come 
together, so that the mantle is produced on each side, lining the 
hemispherical membranous sac, which separates into two halves 
when the valves open, is protected by the shelly dome above and 
by the flat surface of the stone (or shell upon which the parent 
rests) below. In this snug retreat it is probable the eggs are re- 
tained until hatched and the young for an indefinite period. The 
marsupium in all the specimens examined was well filled with 
young fry which had passed the embryonic stages. 
A matter of interest connected with this discovery is the 
evidence it shows of the process by which the more complicated 
marsupium of Thecalia concamerata Ad. (see pl xxiv, fig 8) was 
formed. Hitherto the latter, as far as I recall at present, has been 
the only lamellibranch known in which the outer shell has been 
folded in to form a marsupium. In Milneria the outer layers of 
the shell within the dome remain, and even the evidermis seems 
to persist, indicating that after the young have left their shelter 
the enfolding processes of the mantle may be withdrawn into the 
body of the shell. In Thecalia, on the contrary, the base of the 
dome has become closed by the fusing of the outer layers of the 
shell, the interior of the dome, which has become altered in the 
process to a double funnel (one in each valve) is permanently 
covered by the mantle and secreted by those parts which produce 
only the inner layer of the valves, neither the outer nor the epi- 
dermal layers any longer taking part in its formation. The line 
of fusion from the two sides is plainly marked on the outside of 
the shell of the female Thecalia, the male, as in Milneria, being of 
the ordinary form. Both genera belong to the Carditide, and it 
is difficult not to conclude that in the two forms we have the early 
and the completed stages of a process which has for its end the 
safety of the immature individuals of the species. 
I have written as if the function of the marsupium in Thecalia 
was certain; and indeed I was informed by the late Dr. William 
Stimpson that during his dredgings at the Cape of Good Hope he 
had discovered the eggs in the internal funnels of the female shell. 
This has always been surmised, but the fact of its having been 
actually observed has, I believe, not hitherto been made public 
in print. The specimen figured is one received from Dr. Stimpson 
in 1865. The interest attaching to the study of the reprod:ctive 
