BIVALVES OYSTERS, MUSSELS, ETC. 623 



which there are numerous species, many of them most beautifully coloured and 

 marked. As in the oyster, the mantle margins are double and fringed, but 

 in addition, there is a row of conspicuous round black eyes. Most of the 

 pectens spin a byssus when young, and some do habitually; whilst many not 

 only remain free but move about, progressing through the water by rapidly 

 opening and closing their valves. With few exceptions the Pecten shells are 

 ornamented with radiating ribs. The genus Lima resembles pecten, but is 

 more elongate and not so equilateral. 



The thorny oyster (Spondylus) is ornamented, as its popular name suggests, 

 with spines and other shelly prominences. The Spondyli attach themselves to 

 various objects, sometimes slightly by a few spines, sometimes by the whole 

 surface of a valve. Two curved teeth in each valve so interlock with corre- 

 sponding sockets in the other that it is impossible to separate them without 

 damage. The big Water Spondylus in its growth leaves cavities in the shell 

 substance which retain sea water. 



ORDER IV. EULAMELLIBRANCHIATA. 



In this order is included the great majority of the bivalves. The gills have 

 complete interfilamentary and interlamellar junctions. Both adductors, with 

 few exceptions, are present. The hinge teeth are few and strong. 



The following seven sub-orders may conveniently be distinguished: SUB- 

 MYTILACEA, TfiLLINACEA, VENERACEA, CARDIACEA, MYACEA, PHOLADACEA, 

 ANATINACEA. 



Sub-order 1. SUBMYTILACEA. The Submytilacea are generally devoid of 

 siphons, and hence the pallial impression is simple. They are more nearly 

 related to the Mytilacea than to the Pseudolamellibranchiata just described ; 

 indeed the Dreiasensiidse and the fossil Modiolopsidre are by some included 

 in the Mytilacea, and the Unionidse and ^Etheriid?9 placed close to them. 



The UNIONISE include, amongst others, the fresh-water mussels familiar 

 to all. The young, as soon as they leave the parent shell, attach themselves 

 to passing fish, and, becoming buried in the skin, pass a short period of 

 their lives as parasites. This stage of the young mussel, till its true nature 

 was ascertained, was called Glochidium. 



The ^.ETHERIID.E include sEtheria,, the fresh- water oyster of the Nile, 

 that has a remarkable cellular shell, the layers on the inside looking like 

 blisters. The corresponding South American form, Mnlleria, has only a 

 singrle (posterior) adductor, and looks yet more like an oyster. 



The CYRENID^, another fresh-water family, are represented in the British 

 Isles by the genera Cydas and Pisidium. Formerly there was a third 

 genus, Corbicula, which spread all over Europe and Siberia, but is now 

 confined to warmer climes. 



The Cyrenidoe have equivalve, subtriangular shells with brown periostra- 

 cum. The hinge has two or three cardinals, and laterial teeth are present, 

 which in Corbicula are striated ; the ligament is external. The siphons 

 are short. 



Amongst the marine forms belonging to this group are the CARDITID^:, 

 stout, equivalved, heart-shaped shells, generally with radiating ribs like the 

 Cockles, and having a strong hinge with one or two oblique cardinal teeth 

 and one or two laterals ; the ligament is external. One member of the family, 

 Cardita concamerata, is remarkable for the curious egg-pouch, formed by an 

 infold of the ventral margin of the mantle and shell. The Carditidse are 



