l)L\J PROCEEDINGS OF UNITED STATES NATIONAL MUSEUM. [1885. 



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 dome and closing 

 in below it, thus forming an approximately hemispherical membranous 

 Mac, 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 retained until hatched and the young for an indefinite pe- 

 riod. The marsupium in all the specimens examined was w^ell 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 concamcrata Ad. (see pi. 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 nuirsupium. 

 In Mihieria the outer layers of Uie shell within the dome remain, and 

 even the epidermis 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 withdrawn into the body of the shell.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 lay- 

 ers 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 cov- 

 ered by the mantle and secreted by those parts which produce only the 

 inner layer of the valves, neither the outer nor the epidermal 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 31ilnena, being of the ordinary form. Both 

 genera belong to the Garditidw, and it is diflicult 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. 



1 have written as if the function of the marsupium in Thecalia was 

 certain 5 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 tact 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 studj^ of the reproductive stages in either species can hardly be 

 overestimated, and the attention of observers at the Cape and in Cal- 

 ifornia is hereby respectfully called to the matter. Either species would 

 probably do well in an aquarium. D.] 



Mytijus califoriiiauus Conr. 

 Al»uudant. 



