262 Records of the Indian Museum. [Voi,. XXII, 



periostracal sculpture. Immediate^ above the processes a groove 

 runs transversely across the external surface and from it short 

 longitudinal grooves are given off at right angles and run to 

 the tip of the processes. Above the transverse supramarginal 

 groove and running parallel to it, a convex ridge, varying in 

 breadth and prominence at different stages of growth, can usually 

 be traced. For it I have proposed the name of supramarginal 

 ridge. The margin, including the grooves, is covered with col- 

 umnar ciliated epithelium as far up on the external surface as the 

 lower edge of the supramarginal ridge. Except at a very early 

 stage in free life {Lecythoconcha) the epithelium is degenerate on 

 the ridge, and above it consists of non-ciliate cells. 



The substance of this part of the mantle is composed mainh- 

 of a peculiar kind of connective tissue consisting of polygonal cells 

 with small nuclei, rather thick walls and gelatinous contents, in the 

 main non-protoplasmic. It is cavernous in structure, including 

 numerous ill-defined blood-spaces without cellular walls as well as 

 true blood-vessels. Longitudinal muscles, sometimes powerfully 

 developed, run down the mantle under the external epithelium, 

 and certain oblique strands can also be followed out near the 

 margin, forming a sphincter round the aperture of the branchial 

 chamber. A fine network of muscle fibres also extends inwards 

 from the outer layer. The musculature is much more highlj'^ deve- 

 loped in some genera than in others. 



The nervous system of the margin has not been worked out 

 in detail, but an irregular transverse nerve, some parts of which are 

 nearer the edge than others, runs above the supramarginal ridge, 

 and sends down fine longitudinal strands at intervals to the calci- 

 ferous glands. 



The glands whereby the greater part of the substance of the 

 shell is secreted lie just above the edge of the mantle and are of 

 two orders, the periostracal glands, which secrete the periostracum 

 or epidermis of the shell, and the calciferous glands, which 

 secrete the calcareous matter. The former are true multicellular 

 glands of a vermiform shape, consisting of contorted tubules 

 and opening to the surface by ducts with cellular walls. They 

 lie some distance below the external surface in a transverse series 

 along the extreme margin, for the most part beneath {i.e. distad 

 of) the calciferous glands and with the main axis of each gland 

 at right angles to the margin. Their ducts open into the supra- 

 marginal groove. The calciferous glands are much more bulky and 

 differ considerably in structure. They occupy the supramarginal 

 ridge and as a rule extend slightly beyond it both above and 

 below, lying only a short distance beneath, or actually on, the 

 surface and having no cellular ducts. Like the periostracal glands 

 they form a transverse series, though the main axis of each gland is 

 at right angles to the margin. Each gland is at first an elongate 

 cyhndrical tubule of gland-cells forming a large number of closely 

 adpressed loops in the external margin of the connecting tissue. 

 The cells are large and do not appear to have any very intimate 



