574 
BULLETIN OF THE BUREAU OF FISHERIES 
a gelatinous appearance but is readily split into horizontal right and left layers of fine 
fibers. At each end of the clear portion of the cartilage is a whitish, opaque pad, 
presumably a layer of cartilage impregnated with lime. 
The hardness and brittleness of the scallop shell make difficult the preparation of 
sections for the study of calcareous structure. Dakin (1909), who made sections, 
states that prismatic and nacreous layers can not be definitely distinguished, that an 
irregular arrangement of crystals (chiefly aragonite) representing both these layers 
makes up the calcareous portion of the shell. In addition he found a trace of perios- 
tracum. Jackson (1890) states that the prismatic layer is present in the early dis- 
soconch shell, but wanting in the adult. Drew (1906) found only the nacreous 
layer present in the shell of the giant scallop of commerce. Belding (1910), on the 
other hand, states that both nacreous and prismatic layers are present. 
The present writer has examined numerous shell fractures instead of sections. A 
fracture along a rib of an upper valve ordinarily shows an outer white area, an inter- 
mediate dark area, and an inner white area. A fracture across the ribs shows the 
intermediate dark area divided by narrow, white areas at the edges of the ribs, con- 
necting the outer and inner white areas. (See fig. 5a.) Occasionally a rib is found 
which has no evident outer white 
surface. The inner white surface 
does not extend quite to the tips 
of the dark ribs. 
In any fracture the shell ap- 
pears laminated. When color is 
present there are the inner white 
laminae, the intermediate brown 
laminae, and the outer white lamina. 
No further lamination within this 
Outer white layer has been distin- 
guished, but in some fractures cross 
striae have been seen. In fractures across the ribs the laminae appear approximately 
parallel to the valve surfaces. In fractures along the ribs the brown laminae are seen 
to extend at an angle and toward the ventral margin from the outer white layer to 
the inner white layer. The laminae of the inner white layer are parallel to the inner 
surface. The layer ( Hypostracum or clear substance ) laid down at the end of the 
adductor muscle is relatively transparent and is clearly and regularly cross striated 
in fractures. In positions abandoned by the adductor in its advance, this clear layer 
is overlaid by the inner white layer, and near the umbos of some shells also by 
the brown layer. 
Because of its distinctness from the underlying shell material, it is suggested 
that the generally thin outer white layer represents the prismatic. Seemingly the 
laminated structure, whether lying at an angle with or parallel to the inner white 
layer, and whether secreted within or without the pallial line, must be considered 
nacreous. 4 Because the brown material is found overlapping the clear layer, it is 
evident that the ability to deposit it is not confined to the mantle margin. From 
the slant of the brown laminae, it seems, according to the accepted secretion theory, 
that for their deposition the outer or shell fold must be reflected sharply back over 
4 According to Pelseneer (1906) the nacreous layer is composed chiefly of aragonite, of which, according to Dakin (1909) a scallop 
shell is mainly constituted, See also ITorwood (1911, 1912). 
Figure 5. — Diagrams of shell structure: a. Cross section of rib and 
portion of adjacent grooves of left valve. (The upper white represents 
the outer white area which may be prismatic. The intermediate dark 
area (actually brown) and the inner white area are assumed to con- 
stitute the nacreous layer.) 6, Cross section of shell through groove of 
upper (left) valve, rib of lower (right) valve, and “cartilage box” 
