4°4 
MOLLUSCS. 
without a pallial sinus, and pearly within. The hinge is composed of numerous 
pointed interlocking teeth on each side of the cartilage-pit beneath the umbones. 
Unlike Nucula, the genus Nuculina is provided with two small adjacent siphons. 
The shell is usually somewhat produced or beaked posteriorly, has a slight pallial 
sinus, and is not pearly within. The hinge-teeth and the resilium are as in 
Nucula. Yoldia is like Nuculina in shape, but has longer siphons, and the 
periostracum more glossy. The shells of Malletia are like those of Yoldia, but the 
ligament is external. Nucula and Nuculina have a world-wide distribution, and 
are numerously represented in species. Yoldia and Malletia, on the contrary, have 
comparatively a few representatives in Arctic, Northern, and Antarctic regions. 
The fossil forms belonging to this family are far more numerous than the recent, 
and include several generic groups which no longer exist. In the second family, 
Solenomyidce, the animal is remarkable for its proboscis-like foot, expanded at the 
end into a flattened disc with a dentate edge. The mantle is united ventrally, 
but open in front for the passage of the foot, and posteriorly for the siphons. The 
shell is elongate, compressedly subcylindrical, without hinge-teeth, and clothed with 
a thick dark chestnut-coloured horny periostracum, which, when dry, is very 
brittle. Only about six species of one genus, Solenomya, are known, but these are 
widely distributed, being found in the Mediterranean, on the east coast of North 
America, in Patagonia, the Indian Ocean, Australia, and New Zealand. 
Order Filibranchiata. 
In this group the gills are smooth and their parallel filaments are directed 
ventrally, reflexed, and provided only with ciliated interfilamentary junctions; 
the foot being usually furnished with a byssal gland. In the family Anomiidce 
the shells of the typical genus Anomia are generally very irregular in their growth, 
inequivalve, and somewhat pearly within; 
the more convex valve being remarkable for 
the large number of muscular impressions, 
and the flat valve for a perforation near the 
hinge. This aperture is for the passage of a 
calcified byssus (n), by means of which the 
mollusc attaches itself to rocks and stones. 
The animal has a small foot; the mantle is 
free all round, and there is but a single 
central adductor muscle (m). About forty 
species are known, two of which occur in 
Britain. Placuna is another genus of this 
family, in which the shells are very flat, with¬ 
out any byssal opening; the valves being thin, 
somewhat nacreous, with two long divergent 
hinge-teeth to which the ligament is attached. About half a dozen species from 
the Indo-Pacific Ocean are known. P. sella has a somewhat wavy or cockled 
appearance, and is known as the saddle-oyster, on account of its saddle-like form. 
The arks (Arcidce) are nearly all strong heavy shells, generally equivalve, but in 
'in - 
right side op Anomia, with shell removed. 
a, Opening for hinge ; m. Adductor muscle ; n, 
Calcified byssus. 
