40 



posterior margin, it is much thicker, and both periostracum 

 and organic matrix are present. The internal structure is 

 very pecuHar, differing from that of most lamelhbranch 

 shells, and corresponding closely to what Ehrenbaum,* 

 who has investigated various species of Cardium, describes 

 as the gastropod type of shell structure. The calcareous 

 substance is distributed in two ill-defined layers (fig. 29, 

 PI. v., Sh.i., 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 laminae which lie, 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 laminae to those already formed, 

 its most external layer is formed by the edges of the 

 laminae 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 laminae the directions of the fibres are at 

 right angles to each other. Thus, starting from any one, 

 in laminae 1, 3, 5, 7, the fibres are parallel to each other, 

 but in laminae 2, 4, 6, 8, they a,re 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 laminae. 

 • Zeitschr. f. wissensch. Zool. Bd. XLI., pp. 1 — 47, 1885. 



