no. 1761. I NEW SPECIES OF ONCHIDIOPSTS i:\Ull. 471 



highly specialized. Probably they are concerned in respiration and 

 also furnish an extensible, thick, and soft sort of cushioned armor for 

 the head, which lacks the ordinary defense of withdrawal under a shell. 



Shell.— (PI. 21, figs. 4, 5. 6.) On cutting open the notseum the shell 

 appears as a quite transparent whitish film closely applied like a cap 

 to the top and front of the solid dome of the visceral hump. Not being 

 adherent either to the notseum or to the true mantle (which lies below 

 it as a transparent membrane investing the visceral hump) it can be 

 simply lifted out with forceps. It has about the appearance and con- 

 sistency of a film of collodion. It is not stiff enough to resist the action 

 of gravity in air, but has sufficient elasticity to regain its form when 

 restored to a liquid medium. It is smooth except for rather faint 

 concentric lines of growth. In general shape it may be roughly 

 likened to a very highly arched finger-nail. A peculiar and instruc- 

 tive feature is the infolding of the posterior-inferior portion into the 

 posterior-superior portion. A comparison of PI. 21, figs. 7, 8, and <), 

 will make clear what has happened. Figs. 7 and 8 are rough rep- 

 resentations of the shells of Marsenina prodita (Loven) and M. 

 ampla (Verrill), respectively. The former is lymnoid. The latter 

 is more degenerate and consists of only one whorl, mostly mouth. 

 Fig. 9 is a diagrammatic sketch of our shell represented as a solid 

 object. The homology of the infolded portion is plain. The last 

 whorl, becoming degenerate to the point of abandonment and 

 reduced to a mere, posterior wall of the mouth space, lias simply 

 collapsed into the spire cavity. This is not, however, a mere acci- 

 dental collapse, as wet paper might collapse upon itself. It is struc- 

 tural and permanent. The infold, if straightened out, springs 

 smartly back again and the line of folding is clearly marked struc- 

 turally. The cavity of the last whorl (i. e., the space between the 

 infold and the to}) of the shell) has been abandoned by the animal and 

 practically obliterated. In this and other obvious respects the 

 infold materially differs from the "deck'' or "shelf" of Crepidula, 

 with which, however, it is somewhat parallel. In the other species 

 of Onchidiopsis there is no such structure, so that in this respect the 

 present species serves to connect the genus as hitherto known with 

 more normal forms and oilers an instructive study in the degenera- 

 tion of shell armor. The shell in the present species differs from those 

 hitherto describe* I in present ing neither lateral emarginat ion nor wing. 



Visceral hump. Next below the thin membranous true mantle 

 already mentioned lies the solid smooth dome of the visceral mass, 

 purplish-yellow in color, the visible portion chiefly consisting of the 

 glands of the genital system. It is larger than the shell, which only 

 partially covers it, but the discrepancy is u<>( so great a- in the other 

 species of the genus, where t he shell is lit t le more t ban a scale over the 

 gill-cavity region, while here the whole top and sides of the visceral 

 hump are covered, [n the gill-cavity region the black base of the large 

 osphradium showing through the roof of the cavity is conspicuous. 



