232 MOLLUSCA. 



" 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 Radiolite 

 consists of prismatic cellular structure ; the prisms are perpen- 

 dicular 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 



Spondylus The chief peculiarity of the Hippuri- 



tidce is the dissimilarity in the structure of the valves, but even 

 this is deprived of much significance by its inconstancy. The 

 free valve of Hippurites is perforated by radiating canals, 

 which open round its inner margin, and communicate with the 

 upper surface by numerous 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" 



The shell of Hippurites (fig. 190) is inversely conical or 

 cylindrical, and sometimes attains a length of a foot or more. 



Fig. lai.Cafrina Agnilloni. The right-hand figure shows the interior of 

 the left valve. 



The shell is attached by the larger conical valve, and is closed 

 by a small depressed free valve, with a central umbo. In 



