FAMILY 1. TRIDACNACEA. 131 



Tridacnacea. Pectinacea. 



Mytilacea. Ostracea. 



AviCULACEA. 



Family 1. TRIDACNACEA. 



Testa solida, transversa, sequivalvis, lunula plus minusve hians ; mar- 

 ginibus ventralibus valvarum sinuosis, interclaudentibus. Impressio 

 muscularis duplex aut bipartita, prope ad marginem ample expansa. 

 Animal bysso affixum. 



The Tridacnacea exhibit a well-marked assemblage of characters, and 

 may be readily distinguished. There are but few species, and each 

 of them is intimately allied with the rest ; so much so, that both De 

 Blainville and Deshayes consider a further subdivision of this family 

 unnecessary ; we propose, however, to follow Lamarck's plan of dividing 

 the Tridacnacea into two genera. 



Their shell may be described as being solid, transverse, equivalve, and 

 more or less gaping at the lunula under the umbones ; the ventral or 

 outer margins of the valves being deeply sinuated and interclosing with 

 each other. The muscular impression is duplex or bipartite, situated 

 about the centre, and spreading nearly to the ventral margin. 



The Tridacnacea are generally of large size, living attached to rocks 

 and corals by a strong byssus ; and, as they are generally found in ex- 

 posed situations, are brought to this country in great abundance. 



The following are the two genera into which this family is divided : 



Tridacna. 

 Hippopus. 



s2 



