4io 
MOLLUSCS . 
great depths. The fossil forms, on the contrary, are more abundant, from the 
Carboniferous age, and some of the species from the Lias ( Plagiostoma) are of 
very large size. The members of the allied family Spondylidce are known 
popularly as thorny-oysters, on account of the spiny character of their surface- 
ornamentation. In general shape they are rather like the pectens, and similarly 
brilliantly coloured; but they have much more solid shells, and the hinge consists 
of powerful interlocking teeth, while the animal has no byssus, a more rudi¬ 
mentary foot, and lives, with a few exceptions, attached by one of the valves to 
rocks and stones. The ligament of Spondylus is internal. The single adductor 
muscle is a little excentric, and the mantle-margin has a row of eyes. 
Order Eulamellibranchiata. 
In this order the gills have vascular, interfilamentary, and interfoliary 
junctions; and the mantle is always united at one or more points, and there are 
generally two adductor muscles. The order is the largest of all, and comprises 
nearly sixty different families, of which only the most important or remarkable 
can be mentioned. In the first (Submytilacea) of several suborders into which 
the order is divided, mention may be made of a curious little species of the 
family Carditidce, namely, Thecalia concamerata, which is a native of South 
Africa, and remarkable for a cup-like process formed by the female within the 
ventral margin of the valves, serving as a nursing-pouch for the young. Milneria 
minima, a Californian species, forms a similiar marsupium. In the family 
Cyprinidce, Isocardia cor is one of the finest of the British bivalves; and is a 
large strong globose shell, with the umbones prominently curved anteriorly. The 
ligament is external, and the hinge-teetli are strong and of peculiar form. The 
animal has short siphons, large gills, and a small foot for burrowing in the sand. 
In the Lucinidce the shells are mostly white, round, globose, or compressed, and 
peculiar on account of the great length of the anterior muscular scar, which falls 
within the uninterrupted pallial line. Sometimes the animals have only a single 
branchial lamella, and the foot is generally slender and without byssus. The 
families Leptonidm, Galeommidce, and Cldamydoconchidce also belong to this 
order. Lepton often lives commensally with Crustacea, Galeomma has the mantle 
reflected over a considerable part of the valves, and in Cldamydoconclia the shell 
is wholly covered by the mantle, a unique feature among the bivalves. The family 
yEtheriidce includes a few remarkable bivalves known as fresh - water oysters. 
They occur in the Nile and some other rivers of North Africa, and some parts of 
South America. When young, JEtheria is a freely creeping mollusc, but when 
adult becomes attached to stones and other substances like the oyster. The shells 
are irregular in their growth, and are of an olive-green colour. The somewhat 
pearly interior of the valves is marked with two adductor scars, and the pallial 
line is entire. They may be regarded as irregular forms of Unionidce without 
a foot, modified for a sedentary life. 
The numerous kjnds of fresh-water mussels (Unioniclcc and Mutelidce ) occur 
in the lakes and rivers of all continents, and the large islands of the Malay 
Archipelago and New Zealand; although in most of the smaller islands they 
