16 NILS HJ. ODHNER, MOLLUSCA. 
dark green. Besides these characteristics pointed out by JAMESON, there are observ- 
able discrepancies between the two species also in the shape of the muscular scars 
on the interior side of the shell. "The specimens from Cape Jaubert differ from equal- 
sized ones of P. margaritifera in having the upper part of the muscular scar pro- 
duced and relatively broad, its length equalling a third of the total length of the 
scar, and its breadth, in old specimens, about ”/s of that of the scar. In P. marga- 
ritifera, on the contrary, the upper lobe of the scar is considerably smaller, forming 
in length less than a third of the total length and in breadth about half that of the 
scar. This difference in the scars corresponds to a somewhat different arrangement 
of the muscles of the animal. In P. maxima the foot retractors have been inserted 
at the middle of the adductor, and the upper part of the latter has been produced 
dorsally. In P. margaritifera, on the contrary, the retractors are inserted with their 
chief part above the middle of the adductor, which thus projects beyond them only 
with a small portion. The same shape of the muscular scar is also found in specim- 
ens from KEast Indies, which in other respects exhibit the characteristics of P. maxima, 
so that there is no doubt that the two forms are specifically distinct. 
According to JAMESON, the present species occurs on the north and north-east 
coast of Australia, on the coasts of N. Guinea, Borneo, Philippines, and Salomon 
Islands; W. Australia; (HEDLEY 1916). 
Pteria (Electroma) zebra REEvE (Pl. 1, figs. 6—8). 42 miles W. 8. W., 42 
feet ("/), 2 sps, max. 1. 6.8 mm, on Lytocarpus phoeniceus (BUSK); 45 miles W.S. W., 
60 feet ('/s), many sps, max. I. 8 mm, on Plumularia badia KiIRCHENP., and 1 small 
sp., 1. 3.5 mm in a large colony of Ophiodes dichotomus (ALLM.); 66 feet ("”/1), 1 sp., 1. 11.2 
mm, on Plhwwularia badia; 12 feet ("”/s), many sps, max. 1. 5.7 mm, on Lytocarpus 
phoeniceus. — "The species is figured by REEVE (Conch. Icon. 10, 1857, Avicula, sp. 
36) and, recently, by LYNGE (1909, PI. II, fig. 16—18). It occurs on the Maledives, 
the Gulf of Manaar and the Gulf of Siam, Port Elizabeth, Queensland, and Tasmania 
(LYNGE 1909); W. Australia (HEDLEY 1916). 
It is a very remarkable fact that the beautiful lineation of the specimens found 
off Cape Jaubert accords very closely with the substratum, so that the brown lines 
are narrower and somewhat denser in specimens attached to Lytocarpus (fig. 6) and 
coarser in those on Plumularia (fig. 7). Further, the ground colour of the shell is 
light yellowish in the former specimens, pure opaque white in the latter, where all 
the pigment has been concentrated in the brown stripes, thus giving them a deeper 
brown tint. The distribution of the colour, judging from these two types takes place 
during growth. In both cases the nepionic shell is colourless, and the brown stripes 
do not begin to appear until after attachment. No reciprocal change of colour between 
the two varieties, resulting in specimens on Plumularia with narrow, and in such on 
Lytocarpus with broad, lines has been observed in the present material, nor any 
stages of transition between the two types. This case of variation, consequently, 
seems to be due to the direct influence of the substratum, and the adaptation of 
the mussel to it may be explained by the presumption of the susceptibility of its 
