SIPHONIDA. 495 



The shell of Hippurites (fig. 359) is inversely conical or 

 cylindrical, and sometimes attains a length of a foot or 

 more. The shell is attached by the larger conical valve, 

 and is closed by a small depressed free valve, with a central 

 umbo. In Radiolites the shell is inversely conical, bi-conical, 

 or cylindrical, with dissimilar valves. The upper valve is 

 sometimes flat, sometimes conical, and has a central umbo. 

 In Caprina (often placed among the Chamidce) the valves 

 of the shell (fig. 360) are dissimilar, the fixed valve being 

 conical, whilst the free valve is oblique, or is spirally rolled. 



Fig. 360. Caprina Aguilloni. The right-hand figure shows the interior of the left valve. 



The free valve is thick, and is " perforated by one or more 

 rows of flattened canals, radiating from the umbo, and open- 

 ing all round the margin " (Woodward). The cavity of the 

 free valve is sometimes chambered. Other genera allied to 

 Caprina are Caprinella, Caprinula, and Caprotina. 



FAM. 9. TRIDACNHLE. Shell equivalve; ligament external; 

 muscular impressions blended, sub-central. Animal attached 

 by a byssus, or free. Mantle-lobes extensively united ; pedal 

 aperture typically large; siphonal orifices surrounded by a 

 thickened pallial border. Foot finger-like and byssiferous. 

 The shell is truncated in front, the surface ribbed, and the 

 margins toothed. In Tridacna (fig. 361) itself the opening 

 for the foot (" byssal sinus ") is large, and placed just in front 

 of the beaks ; whereas in the closely allied Hippopus the 



