TEREDO. 87 



This ear-shaped appendage varies in its outlines simihirly 

 to navalis, rising, in the adult, above the dorsal line, and, 

 in the immature shells, sinking far below the level of the 

 beaks, owing to the then greatly declining curve of the 

 dorsal edge. 



There is one fact with regard to the shipworms which 

 has rendered their investigation peculiarly laborious, namely, 

 that no reliance can be placed upon the relative proportions 

 of their several parts for specific definition. If we take 

 at random about fifty valves of norvagica for instance, 

 we shall find that in some the oblique decussated stride oc- 

 cupy twice the space of the succeeding strip, in others this 

 is reversed, in many these are both contracted, and a large 

 posterior smooth area is exhibited ; in others again, almost 

 the entire surface is occupied by the two former, to the 

 great diminution of the hinder portion. Hence it is abso- 

 lutely necessary to examine very numerous examples, in 

 order to elicit the real and permanent specific characters, 

 and the valves alone are rarely adequate for the determina- 

 tion of the species. 



The pallets, which are extremely fragile, and never at- 

 tain to any considerable dimensions, closely resemble dimi- 

 nutive specimens of those of hipennata. They vary much 

 with age and circumstances in regard to the number of arti- 

 culations, their closeness or laxity of approach to each other, 

 and even in their individual shapes. In the smaller speci- 

 mens, (and almost all hitherto taken in our seas belong to 

 this class, not exceeding half an inch in length,) the stem 

 resembles a piece of fine thread, and is about equally long 

 with the broader pinnated portion which surmounts it. 

 This latter is composed of numerous somewhat triangular 

 pieces, of which the narrower end is jointed as it were to 

 the broader opposite extremity of the preceding one, which 



