72 CONCHOLOGY. 



more or less elevated ; posterior slope heart-shaped, and 

 widely gaping. 



Tridacna gigas. Tridacna crocea. 



T. elongata. T. mutica. 



T. squamosa. T. serrifera. 



T. gigas. The giant Tridacna. PL 12, fig. 3. 



Species of which the shell is sometimes very large, white, 

 transversely ovate or elongated ; the anterior side longer 

 than the posterior ; broad ribs covered with vaulted scales ; 

 when of a pink or orange colour, greatly valued. 



2. Hippopus. One species. 



Similar to the Tridacna, but distinguished from it by having 

 its posterior slope closed with a dentated margin ; its ribs 

 are never arched or vaulted, and its anterior side is shorter 

 than the posterior side. 



H. maculatus. The spotted Hippopus. PI. 12, fig. 1. 



Shell transversely ovate, ventricose, with scaly ribs ; lu- 

 nule, heart-shaped, and oblique ; margins deeply crenulated ; 

 reddish purple spots. 



FAMILY XVI. 



Mytilacea. Three genera. 



1. Mytilus. The Muscle. Thirty-five species. 



As arranged by Lamarck, now comprises only such shells 

 as are regular, equivalve, and longitudinal ; solid in sub- 

 stance, and attached to other bodies by a short thick byssus. 



In colour and appearance they greatly vary, some being 

 smooth and beautifully variegated with delicate colours, or 

 radiated with purple and white ; some are coarsely ribbed or 

 granulated, and have only one colour, as black, blue, yellow, 

 brown, or green ; all are covered with an epidermis, to which 

 oftentinnes the colour is confined. 



Shell of a close texture, elongated, more or less oval, 

 sometimes sub-triangular, equivalve, summits anterior, more 



