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 dioecious 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 Areas on a plane surface liVe 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 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 marsupium. In Milneria the outer layers of 

 the 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 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 Carditidae, 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 reproductive 



