250 Records of the Indian Miiseuui. [Vol. XXII, 



The external structure of the processes (l^e they primary, 

 secondary or tertiar}') is identical. They are not mere projec- 

 tions of the margin but organs with a definite form, position and 

 function. When fully expanded in the living animal they are 

 flattened dorso-ventrally and sharply pointed, but it is difficult to 

 preserve them quite in this condition as they usually become blunter 

 and thicker in preservation-;, as they do in life when the mantle 

 contracts (fig 12). Along the external surface of each, from a 

 point close to the tip, runs a narrow groove, and the whole of both 

 surfaces, including the floor of this groove, is densely covered with 

 long and powerful cilia. These extend also all over the edge of the 

 mantle. Very often (fig. 12) the presence of a tertiary, or even a 

 secondar}', process is only indicated by the existence of this groove, 

 which I shall call a marginal groove. 



The marginal grooves run up the external surface of each pro- 

 cess to a broader and rather deeper transverse groove that traverses 

 the whole of the margin just above the bases of the processes and 



Fig. i,^. — l^iviiig Lecvthoconclia lecvfliis a fortnight old (magnified). 

 m., edge of mantle; i, 2, 3. primary rows of chaetae ou shell; i,' 2,' 3', 

 primary marginal grooves, 



turns upwards for a short distance at the right extremity of the 

 free edge. This groove I shall call the supramarginal groo\te. 



Immediately above the supramarginal groove on the external 

 surface of the mantle is a broad and prominent ridge, which has 

 been called the " white band " on account of its lack of epidermal 

 pigment, but may be also known as the supr.'Vmarginal ridge. 

 Its surface is smooth and ciliated at birth in all the species exam- 

 ined at this period, and in Lecythoconcha its limits are not clearh* 

 indicated. 



In the foregoing paragraphs I have described the external struc- 

 ture of the marginal region as it exists at the birth of the young 

 mollusc, it remains to be seen how far it alters in the course of 

 post- embryonic development. There is greater specific and generic 

 variation in this respect than there is in the primitive structure, for 

 while in some species and genera the marginal processes are greatlj- 

 reduced in the adult, in others they retain their primitive condi- 

 tion, while in yet others they increase in proportionate develop- 

 ment. I was under the impression that they disappeared com- 



