FEMALE OR HERMAPHRODITE. 255 



ridge running from the apex to the tergo-lateral angle. 

 The lines of growth do not end abruptly at the tergo-lateral 

 angle, as is the case with S. ornatum and several fossil 

 species, but run up a little way along the tergal margin. 

 The umbo is seated at the uppermost point, and, there- 

 fore, the main growth is downwards. There is a large 

 rounded depression for the adductor muscle (a, fig. 2 a), 

 and higher up, opposite the tergo-lateral angle, there is 

 another hollow (b), for the lodgment of the males ; this 

 latter is of nearly the same shape as the hollow for the 

 adductor musle, but rather more conspicuous than it. 

 From the appearance of the under surface of the scuta, it 

 might read ly have been thought, that there had been two 

 adductor muscles. 



Terga, of large size, longer than the scuta, flat, trian- 

 gular, with the whole inferior part much produced and 

 spear-like. A portion of the apex, must have projected 

 freely above the sack. 



Carina (PI. VI, fig. 2 b'\ simply bowed (i. e., not rec- 

 tangularly bent), with the umbo (and primordial valve) 

 seated at the upper point ; rather massive, narrow, only 

 slightly increasing in width from the upper to the lower 

 end ; the two sides are flat, and at right angles to the 

 roof, which is bordered on each side by a rather broad, 

 square-topped ridge (see section fig. 2 c)\ or the roof may 

 be said to have a square-edged furrow running from the 

 apex to the basal margin, and widening downwards; 

 these two ridges have their lines of growth oblique, and 

 hence have a twisted appearance; the central depressed 

 portion of the basal margin, which is square or truncated, 

 descends lower down than the two ridges. The sides of 

 the valve close to the apex are broad, and consist, as I 

 believe, of intra-parietes, as well as of parietes, but these 

 parts are not separated from each other by ridges, as is 

 commonly the case, more especially with the fossil species. 

 I have described the carina in some detail, on account 

 of its resemblance to that of the cretaceous 8. fossula, 

 S. trilineatum,, and S. quadricarinatum. 



