THE MOLLUSCA HEDLEY. 



509 



The brachial valve is externally horse-shoe shaped, and has a 

 slight median boss. Internally it has a straight hinge line, from 

 beneath which and in the plane of the valve, projects a stout 

 cardinal process, whose transverse vertical section would form an 

 omega, hollow downwards. On either side of the cardinal process, 

 and corresponding to the teeth of the lower valve, are two deep 

 triangular impressions, the sockets. All the free edge of the 

 upper valve is granulated. The frontal emargination gradually 

 passes into a funnel directed backwards here originates the 

 median septum which tapers distally to an acicular point before 

 the hinge. The ventral face of the septum is hollow, on the right 

 and left of it are produced curled flanges with serrate edges. 

 These edges vary much ; in some, presumably old, individuals 

 they project irregular jagged lobes into the cavity. 



Fig. 57. 



If this median septum be compared to the tongue, then the 

 teeth of the human jaw would answer in position to the lateral 

 lobes of the brachial lamellae. Their development varies much ; 

 what I take to be a young stage is shown in my drawing. In 

 other, presumedly aged examples, the " canines " and "molars" 

 project as tusks sideways and downwards, while the " incisors " 

 coalesce and advance towards the hinge. The cavity of the 

 valve, exclusive of the septum and lamellae, has the surface 

 densely perforated. 



This species was attached in considerable numbers, horizontally, 

 perpendicularly, or obliquely (Fig. 57) to loose sheets of dead coral 

 which I pulled up by tangles in forty to eighty fathoms on the 

 western slope of Funafuti. At iirst inspection I mistook them 



