'Z2 POPULA.E, BRITISH CONCHOLOGY. 



various means to secure and protect their bodies and their 

 shells, and make their holes fit them by filling up the in- 

 terstices. Some species will protect the umboes and dorsal 

 edges of the valves by one, two, or more shelly plates; 

 others, by horny coverings in back and front ; others, by 

 extending the edges of the shells themselves, so as to fill up 

 the parts which were gaping. In a new genus of P/ioladiday 

 which I have recently described under the name of Triomp/ia- 

 lia, the right valve increases in length at the hinder end, so 

 as partly to fill up the narrower part of the tube, while near 

 the hinge of the left valve commences, in what I call a third 

 umbo, an inflated shelly covering, which joins the gaping edge 

 of the valve, and overlaps the other valve in front. In the 

 Pholas jjap^racea and some other species, there commences 

 at the posterior end of the shell, an invention for filling up 

 part of the hollow where the tubes of the animal pass, which 

 consists of a kind of cvp or the beginning of a tube. In 

 some species this is of shell, in others it is pure horn, and 

 there are one or two species where it is prolonged into a 

 distinct shelly tube. I may also mention in this place the 

 specimen of Ph, calva figured in the Proceedings of the 

 Zoological Society, in which, besides a very large propor- 

 tion of the usual muniments, the upper part of the hole is 



