184 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 
posterior margin, it is much thicker, and both periostracum 
and organic matrix are present. The internal structure is 
very peculiar, differing from that of most lamellibranch 
shells, and corresponding closely to what Hhrenbaum,* 
who has investigated various species of Cardiwm, describes 
as the gastropod type of shell structure. The calcareous 
substance is distributed in two ill-defined layers (fig. 29, 
Pl. V., Sh.t., Sh.e.), which must be termed inner and 
outer shell layers since the terms prismatic and nacreous 
layers are not applicable here. The shell is composed of 
a great number of exceedingly thin lamine which lhe, for 
the most part, parallel to the shell surface. But since the 
mantle edge is folded over the shell edge, each lamina 
begins as a curved plate, the convexity of which is turned 
towards the margin, and since the whole shell grows by 
the addition of successive laminz to those already formed, 
its most external layer is formed by the edges of the 
lamine coming out on the surface at an angle of from 45° 
to 60°. The deposition of calcareous matter seems to be 
effected principally by a rather wide zone of the external 
surface of the mantle, extending back from the margin. 
Hence the dorsal parts of the shell are thin, since there 
seems to be little, if any, formation of lime over the general 
mantle surface. 
Each lamina has a very fine fibrous structure, the fibres 
lying longitudinally in the thickness of the layer. In any 
two successive lamine the directions of the fibres are at 
right angles to each other. Thus, starting from any one, 
in lamine 1, 3, 5, 7, the fibres are parallel to each other, 
but in lamine 2, 4, 6, 8, they are at right angles to the 
fibres in the first set. This is not easily observed in 
sections perpendicular to the surface of the shell and to 
the margin, owing to the excessive thinness of the lamine. 
* Zeitschr. f. wissensch. Zool. Bd. XLI., pp. 1—47, 1880. 
