KUNGL. SV. VET. AKADEMIENS HANDLINGAR. BAND 52. wN:o [6. 17 
epithelium to light influences and a faculty of reacting on them in such a manner 
that the pigment is distributed according to the distribution of light and shade in 
the closest surroundings. This faculty of reacting is to be regarded as a normal 
characteristic of the present mollusc. That a single specimen has acquired an aber- 
rant colouration (fig. 8) may be explained on the supposition that it has remained 
unaffected by its surroundings; its epithelium has been »blind» and the pigment 
has been diffused uniformly. The occurrence of this specimen, which may be con- 
sidered as badly adapted in comparison with the others, does not speak in favour of 
the view that natural selection in this case was the regulator of the variation. 
The present form would be an interesting subject of experiment with respect to the 
theories mentioned, 
Avicula straminea DunkrrR. 45 miles W. S. W., 60 feet (!*/7), 1 sp., 1. 12 mm 
from a colony of Ophiodes dichotomus (ALLM.); 72 feet (1%/:), 1 sp., 1. 50 mm. The 
specimens agree very well with the description and the figure given by DuNKER 
(Martint & Cuemnitz, Conch. Cab. 7:3, 1872, Tab. 9, Fig. 1). It is distributed in 
the Indian Ocean (DUNKER). 
Perna vulsella Lamarck (Pl. 1, figs. 9, 10). 42 miles W. 8S. W., 70—72 feet 
(*°/s), many sps, max. |. 52 mm; 45 miles W. S. W., 60—80 feet, many sps, max. 1. 68 
mm. The specimens differ from the Madagascar ones in their less obvious radiating 
rays and their more distinct, often strongly developed, posterior wing and sinuous 
posterior margin, which gives them some resemblance to P. fimbriata Rrnve (Conch. 
Icon. 11, 1858, fig. 23). Both the shape and the colour are, however, subject to 
great variation, and as the soft parts of the animals do not exhibit any differences, 
both the Madagascar and the Australian form are here referred to the same species. 
The distribution of the species comprises Natal, seas of India and America (cf. 
Bartscnw 1915); Criesstn 1891 (Martrnt & CHEmnitz, Mallacea) further records it 
from West-Africa (Loanda), and Metyiti 1909 from Mauritius and the Philippines. 
Malleus regula Forskar. 42 miles W. S. W., 72 feet (°/s), 1 sp., 1. 90 mm; 45 
miles, 70 feet, 1 sp., 1. 14 mm, The species is easily recognized by its longitudinal 
rib on the inside of the shell. Distribution from the Red Sea and the Persian Gulf 
to the Philippines, Japan, and 8. Australia (LyNGr 1909); W. Australia (HEDLEY 1916). 
FAM. MYTILIDAE. 
Mytilus horridus DunKkrrR. 45 miles W. S. W., 60 feet (?%s), 1 sp., 1. 70 mm. 
Its form agrees best with J. tortus of Rrrve (Conch. Icon. 10, 1857, fig. 6), which 
EK. A. Smirn 1885 considers identical with M.horridus, and has the umbones turned 
to the left in relation to the posterior shell portion, further the whole shell (not 
only the posterior half, as in Rerrve’s figure) is covered with thick epidermal hairs, 
short at the umbones, prolonged towards the hinder margin, which are dentated in 
their ventral edge. Its long dorsal margin somewhat recalls a Pteria. — The species 
has been recorded from Cape York and the Philippines (E. A. Smrru 1885). 
K. Sv. Vet. Akad. Handl. Band 52. N:o 16. 3 
