KUNGL. SV. VET. AKADEMIENS HANDLINGAR. BAND 52. N:o IG. 35 
height from superior to inferior margin 21.g8 mm, breadth 15 mm; breadth of the 
attached area of the inferior valve 11 mm; breadth of the reflected (free) part of 
the same valve 12 mm; crassitude of the mussle (valves closed) 15.5 mm. 
Locality: 42 miles W. S. W., 60: feet ("/7), I sp., h. 21.8 mm; 45 miles W. 8. 
W., 72 feet ("/,), 2 sps, from Ascidiae and from a Meleagrina, h. 13 (with animal) 
and 7.5 mm (dead). 
Animal (fig. 5) with the mantle closed all round except the short siphons; gills 
two on each side, the posterior one with its descendant lamella reduced and only 
the reflected one retained, highly expanded, with its superior margin free; the inner 
gill with both lamellae well developed; in the large specimen both pairs of gills were 
completely free of each other and of the mantle posteriorly; in the smaller one they 
were joined to each other behind the foot and fused with the edges of the mantle. 
Foot triangular, very short, laterally compressed, with a small cutaneous lobe behind; 
labial palps elongated linguiform, not nearly reaching the middle of the adductor. 
The large shell was attached to a Cardita, the sculpture of which is repeated, 
not only on the inside of the lower valve but also on the outer side of the upper 
one, thus causing here a false or occasional sculpture in the form of a coarse rugation 
that is wholly absent from the smaller specimens. Also in the hinge struction, the 
large specimen is aberrant, inasmuch as the tooth of the left valve has been nearly 
entirely reduced, which has been compensated by a strong development of the socket 
margin in the right valve and an irregular rifling on its upper side, thus causing its 
striking similarity to a true tooth. 
In general appearance the present shell reminds one strongly of a member of 
the family Chamidae, especially of the genus Pseudochama, in respect of its attach- 
ment with the right valve. »Chama fimbriata»> of REEvE (Conch. Icon. 4, fig. 41) 
which is recorded from Port Cunningham and Geraldton (cf. HEDLEY 1916) is much 
similar to it, but the description is too uncomplete to allow identification. The 
presence of an interior ossicle and the aberrant construction of the hinge make it, 
however, evident that the form before us is to be referred to the genus Chamostrea, 
which has hitherto comprised only one species, Ch. albida LAMARCK, from the east 
coast of Australia. The new form differs distinetly from the type of the genus in 
the presence of plicae in the interior of the left valve, as well as by its smaller 
size. With regard to the soft parts of the animal, the new form shows some other 
differences from the type in its gills being free of each other (Chamostrea albida 
has them united posteriorly),' and by the absence of a pedal opening as a conse- 
auence of the complete coalescence of the mantle margins in front of the siphons. 
Basing themselves on the shell characteristics, the systematists have included 
the genus Chamostrea (generally made the representative of a distinct family) either 
with the group Ånatinacea, on account of the struction of the ligament, or with the 
group Chamacea, with respect to its similarity in the outer habitus and its mode of 
attachment, with forms of the family Chamidae. Tt is true that among this family 
! This condition may perhaps be subject to individual variation, as is the case in the forms of Chamidae. 
