THE SURFACE IN BIVALVES. 
49 
inner portion, which combines two modes of striation, at right angles 
to each other, and giving the laminate appearance of nacre with the 
columnar appearance characteristic of the prismatic layer, and thus 
apparently showing the morphological and developmental similarity 
of the ligament and shell. 
The External Surface of bivalve shells is generally more or less 
smooth, but all are marked by the successive growth-lines, which 
indicate the stages of increase in the size of the shell, as each of these 
lines was at one time the actual margin of the valve. The growth 
of a bivalve shell is not uniformly rapid in all directions, the rate of 
increase being most rapid towards the ventral and posterior margins, 
hence the umbo is always close to the dorsal margin and usually 
nearer to the anterior than to the posterior end; the incremental lines 
being more or less prominent according to the species. In some forms 
as Pisidium amnicum the shell is deeply and widely sulcate concen- 
trically, while in others, as the various species of Pisidium and 
Sjdiwrium the periostraca may be clothed with short, stiff, and more 
or less nnmerous microscopic hairs. Those on Sp/iwrium corneum are 
.short, thick, and somewhat bent at the points, somewhat .sparingly 
distributed, but most numerous and thickly set near the umbones, 
and do not appear to be arranged in any set or geometrical order. 
The Internal Surface of the valves is ordinarily whitisli, bril- 
liant and often iridescent, but is sometimes delicately or richly 
tinted with salmon, rose, azure or other colour, and occasionally in 
some species is found of a deep livid blue or blui.sh-purple. 
M. Picard has detected and recorded the presence of a broad ridge 
or thickened part of the shell in the interior of the Uniones, arising 
from the umbonal region and somewhat obliquely traversing the shell 
towards the ventral margin, this elevated area he has named the 
Ventral Crest, and the two divisions or chambers thus formed in 
each valve he has termed the Anterior and Posterior Chambers 
respectively, according as they occupy the antewor or posterior end 
of the shell. 
Colour is usually invested in the epidermis or periostracum, and 
is generally somewhat uniform, though of a stronger and richer tint 
towards the posterior end, the tinting being .said to be greatly in- 
fluenced by the nature and composition of the inhabited water, and 
to be more brilliant and elegant in still waters where there is a thick 
I) 
