MYTILACEA. MOLLUSCA. MODIOLA. 125 



In young and entire shells the hinge margin rises in a straight line 

 to a considerable height, and then slopes downwards, suddenly form- 

 ing a conspicuous angle ; but in older shells the angle disappears in a 

 great measure, and the whole superior outline is regularly curved. 

 When young, the epidermis seems to be prolonged at the lines of 

 growth, into fringe-like shreds. Specimens thus clothed, are generally 

 allowed to be the My'tilus larhatus of Pennant. M. Gilhsii is said 

 to differ in having these shreds serrated or gashed along one edge. I 

 cannot but strongly suspect, though I cannot demonstrate it, that this 

 apparent extension of the epidermis is a parasitic vegetable ; and that 

 M. G'lhhsii is not, in reality, a diflerent shell, but has a ditferent 

 vegetable growing upon it. 



Old shells are also encrusted with various species of 3Iadrepore, 

 Cordllina^ and Flustra. 



In young shells there is usually a broad, waxen-yellow radiation 

 from the beak along the front side of the elevated ridge ; and this re- 

 gion, in fact, always has a lighter color than other parts of the shell. 



Deshayes thinks it is now impossible to say what was the true 

 My'tilus modiolus of Linnaeus, and therefore approves the course of 

 Lamarck in dropping the name altogether, and assuming a new one. 

 I cannot see any reason to doubt that the shell under consideration 

 was the M. modiolus of Linnreus, while there is ground to question 

 whether the shell which Lamarck had in view, when he applied the 

 name Papuuna, the name now universally applied to our shell, was in 

 reality identical with our species. I have seen several specimens of 

 the East Indian shell, and, though very closely allied, it seems to differ 

 in many particulars when the two shells are placed side by side. Dr. 

 Loven has lately assured me that this is the true M. modiolus, and 

 repeats my conjectures as to the distinctness of the real M. Papuana. 



The animal is of a dark orange, or red-ochreous color, perhaps 

 a little tinted with brown. It is not used for food with us, though there 

 seems to be no reason why it should not be as palatable as most of 

 the shell-fish that are eaten. 



Modi OLA plicatula. 



Shell oblong, falciform, widening posteriorly ; surface traversed 

 by numerous radiating ribs, occasionally branching ; epidermis 

 glossy, green and yellow. 



Figure 81. 



