224 SCALPKLLUM VULOARE. 



umbo, or centre of calcification, is seated close to the 

 occludent margin, and at about one fourth of the length 

 of the valve from the apex. Internally, (fig. 15, a, 

 PI. V,) the part above the umbo is flat ; and beneath this 

 upper part, there is a large rounded hollow (d) for the 

 adductor muscle : a fold or indentation (a) running down- 

 wards from the umbo, extends in a very oblique line 

 across the occludent margin. This fold is of high inte- 

 rest as giving lodgment to the Complemental Males, and 

 will hereafter often be referred to. 



Terga, triangular, flat ; occludent margin, very slightly 

 arched. 



Carina much bent, with the umbo placed at barely 

 one third of the entire length of the valve from the apex. 

 Two very slight ridges can be perceived, one on each side, 

 running from the umbo to the basal margin, and sepa- 

 rating the roof from the parietes of the valve ; these 

 ridges are of great use in distinguishing the fossil carinae 

 of Scalpellum, from the carinas of Pollicipes. The part 

 above the umbo is formed by the upward production of 

 a marginal slip along each side of the valve, which slips 

 in the fossil species (C in the woodcut, fig. 1, given in 

 the Introduction,) I have designated as the intra-parietes. 

 The lower part of the valve gradually widens from the 

 umbo downwards ; internally, the whole is deeply con- 

 cave, and continuously curved. The angle varies at 

 which the upper and lower portions externally meet each 

 other; bnt is never less than 135°. . The upper part of 

 the carina runs up between the terga for three-quarters 

 of their length ; the basal margin does not extend down 

 low enough to pass between the carinal latera. 



Mostrum, (fig. 15$', seen externally, and highly magni- 

 fied,) minute, almost hidden by the enveloping membrane 

 and by the small prominent umbones of the rostral latera ; 

 in area equalling about one fourth of the rostral latera ; 

 externally pyramidal, with the upper side rather longer 

 than the lower ; internally slightly concave, square, 

 with the upper margin and sometimes with the lower 



