732 



LAMELLIBRANCHIATA. 



Here also must be placed the remarkable Cretaceous genera 

 Caprina, Plagiopty chits ( = Caprina in part), and Ichthyosarcolites 

 ( = Caprinella and Caprinula), in all of which the shell (fig. 6 1 1 ) 

 is very inequivalve, thick-walled, and either fixed by the apex of 

 the right valve, or, in some cases, free. In all these forms the 

 shell-structure consists of a thin external prismatic layer, and a 

 greatly developed internal laminated or porcellanous layer, the 

 laminae of which may be more or less extensively separated by 

 vacant spaces (the so-called "water-chambers"). In the substance 

 of the left valve in these genera there is, further, developed a 



Fig. 611. — Plagiofttychus {Caprina) Aguilloni. The right-hand figure shows the 

 interior of the left valve. 



system of radial canals which run from the beak to the free 

 margin of the valve, where they terminate in foramina, and which 

 are sometimes simple and sometimes complex in their arrangement. 

 Family 2. Rudist^ {Hippuritidce). — In this family the shell is 

 very inequivalve (fig. 612), unsymmetrical, massive, the two valves 

 being dissimilar in structure and sculpturing. The shell is attached 

 by the apex of the elongated right valve, which is conical in form ; 

 while the left valve is depressed, often operculiform, and has a 

 central umbo. There is no ligament ; but the free upper valve is 

 fixed into the conical lower valve by powerful teeth and processes, 

 and is only capable of movement in a vertical direction. There are 

 two large adductor impressions, those of the left valve being upon 

 prominent apophyses. The pallial line is simple and submarginal. 



The shell-structure in the Hippuritidce is exceedingly peculiar, and has 

 been described by Zittel as follows : " The lower valve consists of two 

 layers, the outer of which is formed of perpendicular prisms having a 

 direction parallel to the long axis of the shell, and intersected by numer- 



