MALE. 251 



of the Males. — These extend nearly parallel to the tergal 

 margin, transversely across the valves, for three fourths 

 of their width ; they are seated above the depression for 

 the adductor muscle, and are more conspicuous than it; 

 they are deep and well defined, and each exactly contains 

 one male. The males are placed with their orifices in a 

 little notch in the occludent margin, and their prehensile 

 antennae at the further end. The distance to which the 

 cavities extend across the valve, and their distance from 

 the upper or tergal margin, varies a little, but chiefly 

 in accordance with the age of the specimens ; for the valve 

 continues to increase in width, whilst the size of the 

 cavity remains the same. The occludent margin of the 

 scutum in the largest female, was '1 of an inch in length ; 

 of another, in which there was a fully developed cavity, 

 *084 ; of a third, in which there was no cavity, only a 

 slight concavity, with a preparatory impression, the length 

 of the occludent margin was *062. The larger and 

 smaller of these three valves, are drawn of their proper 

 proportional sizes, in PI. VI, figs. 1 5', 1 c. The pre- 

 paratory impression (fig. 1 c, b), consists of a narrow, not 

 quite straight, extremely slight furrow, of slightly irregular 

 width, bordered on each side by a very minute ridge, 

 which is distinctly continuous with the inner edge of the 

 occludent margin, both above and below the cavity. The 

 furrow appears to have been formed by calcareous matter 

 not having been deposited along this line, during the 

 thickening or growth of the internal surface of the valve : 

 I suspect, that it originates at a single period of growth, 

 for I could see no signs of successively-formed transverse 

 lines. I believe that it is strictly homologous with the 

 fold, over which the complemental male is attached in 

 8. vulgare, but carried, for a special purpose, much further 

 across the valve and rectangularly inwards, for in structure 

 and position both are identical. In comparing the internal 

 views of the scuta in S. vulgar e and S. ornatum (PI. V, 

 fig. 15 a\ and PI. VI, fig. 1 c), it must be borne in mind, 

 that the latter should be compared, as clearly shown by 



