4 io 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, w r hich 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-teeth 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 Leptonidce, Galeommidce, and Chlamydoconchidce also belong to this 

 order. Lepton often lives cornmensally with Crustacea, Gcdeomma has the mantle 

 reflected over a considerable part of the valves, and in Chlamydoconcha the shell 

 is wholly covered by the mantle, a unique feature among the bivalves. The family 

 ^Etheriidce 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, ^Etheria 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 kinds of fresh- water mussels ( Unionidce 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 



