Geological Societij ((ml A. J. J. S. JfeefiiH/s. — l^/)/i((ni. 253 
necessary to distinguish a median region running transversely. The 
terms ventral and dorsal regions can be more accurately limited than 
in the specialized forms with dorsal hinges. These lie on either side of 
a line drawn from the })eaks, following the curvature of the growth to 
the points above the terminations of the gills; and this line is very of ten 
marked, as in other Pelecypods, by a ridge or elevation on the valve. 
The wings, therefore, are not anterior and posterior, as usually de- 
scribed, but are strictly ventral and dorsal wings. 
The term area is used in all subdivisions of rexjions. and these are of- 
ten well marked by angles or sinuosities in the bands of growth on the 
exteriors and margins of the valves. In the Pholadidfe the areas are 
easily distinguishable: but in many shells two or more areas may run 
together and are not distinct, -a fact of the greatest importance in de- 
scribing some shells, since the areas and their V)lending accurately corres- 
pond to differences in the arrangements and proportions of the jjarts of 
the internal anatomy. 
For the space from the anterior end of the cardo to the depression 
made by the foot, the name oral area is proposed. The depressions, 
when they occur, are the y^erfai sinxses on the margins and pedal de- 
pressions on the exterior: and the succeeding elevation becomes the el- 
evation of the pyedal area. This name is open to the objecticm that there 
is no foot in Ostreidaj, and possibly it might be wise to substitute the 
term corporeal area. 
Beyond this area posteriorly comes the branchial area, marked oflt" by 
depressions in the surfaces and sinuses on the margins. This area usu- 
ally corresponds with the umbonal region in all the Pelecypoda, and as 
a rule the axis of greatest growth of the valves lies in this ax'ea on ac- 
count of the hypertrophy of the gills, the feeding and breathing organs 
of these animals. The deyjressions can be described as the hranchio- 
2)edal depi^essions, and the sinuses as the hranchio-ped((l sinuses. The 
branchial area proper is the space between this and the area occupied 
by the siphons or siphonal ojjenings. 
The sip)honal area is sometimes quite distinctly marked otf from the 
branchial, and has often, as in the Mactrida3, a sharp sijjhonal crest. It 
would be advisable to restrict the use of some terms, like crest, costae, 
elevation, depression, if this terminology finds favor with anyone, to ra- 
diating structural modifications of the surface, and the terms ridge and 
striie, like the lines and bands of growth, to markings parallel with the 
latter. The bands of growth usually make an angle in cro.ssing the si- 
phonat area, and sometimes another as they bend dorsad to the poste- 
rior boundary of the cardo. These are the ventral and dorsal angles of 
the siphonal area: and the crest, where it exists, ie always made Vjy the 
hypertrophy of the ventral angles. It is often of advantage to distin- 
guish two parts in the siphonal area, the incurrent or inlialent part 
ventrad of the siphonal angles or crest, and the excnrrrnt or cloacal 
part dorsad of this structure. 
The intestinal area occupies the space between the dorsal angle of 
the siphonal area and the posterior termination of the cardo. 
