494 



LAMELLIBRANCHIATA. 



of all fossils ; there are no recent shells which can be sup- 

 posed to belong to the same family; and the condition in 

 which they usually occur has involved them in greater ob- 

 scurity. The characters which determine their position 

 amongst the ordinary Bivalves are the following : 

 " 1. The shell is composed of two distinct layers. 

 " 2. They are essentially unsymmetrical and right-and- 

 left valved. 



" 3. The sculpturing of the valves is dissimilar. 

 " 4. There is evidence of a large internal ligament. 

 " 5. The hinge-teeth are developed from the free valve. 

 " 6. The muscular impressions are two only. 

 " 7. There is a distinct pallial line. 



" The outer layer of shell in the Hippurite and Eadiolite 

 consists of prismatic cellular structure ; the prisms are per- 

 pendicular to the shell-laminae, and 

 subdivided often minutely. The 

 cells appear to have been empty, 

 like those of Ostrea. The inner 

 layer which forms the hinge and 

 lines the umbones, is sub-nacreous, 

 and very rarely preserved. . . . 

 The inner shell -layer is seldom 

 compact, its laminae are extremely 

 thin, and separated by intervals 

 like the water-chambers of Spon- 

 dylus. . ,. * The chief pecu- 

 liarity of the Hippuritidce is the 

 dissimilarity in the structure of 

 the valves, but even this is de- 

 prived of much significance by its 

 inconstancy. The free valve of 

 Hippurites is perforated by radiat- 

 ing canals, which open round its 

 inner margin, and communicate 

 with the upper surface by nume- 

 rous pores, as if to supply the interior with filtered water. 

 In the closely allied genus Radiolites there is no 

 trace of such canals, nor in Caprotina." 



Fig. ^.Hippurites 2"oucasiana. 

 A large individual, with two smaller 

 ones attached to it. 



