ON THE STRUCTURE OF ENIGMA ^NIGMATIOA. 283 



the byssus cavity of the two genera, aud I have been unable 

 to find any accurate drawing or description of the minute 

 structure of the byssus of the Anomiacea. The " ossicle " 

 being absent in Enigma, the byssus and byssus-gland of this 

 genus is a more suitable object for microscopical study than 

 the corresponding organ in Anomia, and its resemblance to 

 the byssus gland of Area tetragon a as described by 

 Boutan (4) is obvious. In the latter genus the foot is very 

 small, and has a groove on its hinder or posterior surface ; 

 the byssus cavity is very large, and the byssus is a stout oval 

 structure consisting of a number of lamellce which, as Boutan 

 says, overlie one another like the coats of an onion. The 

 byssogenous gland consists of some twenty to twenty-five 

 parallel epithelial folds or laminae, which traverse the interior 

 of the byssus cavity, aud the chitinoid lamellce of the byssus 

 are formed from, and their inner ends are contained between, 

 these laminae. Following Boutan, I will describe the internal 

 divisions of the byssus itself as ''lamella" and the epithelial 

 folds of the byssogenous gland as 'Maminse." 



The extent of the byssus-cavity in Enigma and its 

 relation to the massive byssus-muscle are shown in the small 

 scale drawing fig. 10, and a transverse section through both 

 byssus and byssus-gland as seen under a higher magnification 

 in fig. 17. The left-hand part of the latter figure shows that 

 the lamella of the byssogenous gland are folds of the epi- 

 thelium lining the byssus cavity, each fold having in its 

 centre a core of connective tissue. The figure, though drawn 

 under a magnification of 600, is on too small a scale to show 

 histological details clearly, but it may be seen that the edge 

 of each lamina is covered by large columnar cells filled with 

 granules. To the right of the figure the byssus itself is seen 

 in situ. It consists of a number of chitinoid lamellce lying 

 between the byssogenous epithelial laminae. The dark line 

 down the centre of each lamella shows its double origin from 

 the walls of adjacent laminae. The outer ends of the lamellae 

 are united to form a plate, and it is evident that this plate 

 increases in thickness by the addition of material secreted by 



