MARINE MOLLUSCS 



199 



which contains a number of species that exhibit very remarkable 

 features both as regards structure and habit. The shell is very 

 thin and fragile, but yet composed of hard material, and its surface 

 is relieved by a series of prominent concentric ridges that bear a 

 number of little rasp-like teeth. It gapes at both ends, has neither 

 true hinge nor ligament, and is often strengthened externally by 

 two or more extra or accessory valves. The hinge-plate is a very 

 peculiar structure, for it is reflected over the exterior of the um- 

 bones, above which they are supported by about ten thin shelly 

 plates, the whole thus forming a series of chambers. The accessory 

 valves are supported by these bridged structures, and a long, 



FIG. 132. Pholas dactylus 

 I, ventral aspect, with animal ; 2, dorsal aide of shell showing accessory valves 



straight, calcareous plate also fills the space along the dorsal side 

 of the shell in some species. The muscular scars and the pallial 

 line are distinctly seen on the inner surface, and a peculiar curved 

 shelly plate projects from under the umbo of each valve. 



The animal inhabiting the shell is somewhat wormlike in 

 general form, and the mantle lobes are united in front that is at 

 the lower end of the shell as it lies in the burrow except that an 

 opening is left for the protrusion of the short foot. The siphons 

 are united and much elongated, so that they protrude beyond the 

 mouth of the burrow when the animal is active ; the gills are 

 narrow, and extend into the exhalent siphon ; and the anterior 



