SCALPELLUM. 17 



but in two or three species are marked with longitudinal ridges ; they are generally rather 

 thin ; this, however, is a character which is variable even in the same species. 



Carina narrow, widening but little from the apex downwards, shghtly or considerably 

 curved inwards, with the umbo, seated at the uppermost point : S. ma(jnum, however, 

 must be excepted, for in it the umbo is sub-central, and the valve almost angularly bent, as 

 will be described in detail under that species. The apex rarely projects freely ; but 

 this is a variable point in the same species ; the basal margin is either pointed, rounded, or 

 rarely truncated. The chief character by which this valve can be recognised, as belonging 

 to the genus Scalpellum, is the distinct separation by an angle, (see woodcut. Fig. 1, in the 

 Introduction,) often surmounted by a prominent ridge, of the tectum or roof, from the 

 parietes, which are either steeply or rectangularly inflected ; the lines of growth on these 

 parietes are oblique. A still more conspicuous character is afforded by the part (when 

 present), which I have called the intra-parietes ; these give to the valve a pieced appearance, 

 and seem let in, to fill up a vacuity between the upper part of the carina and the terga, 

 and this is their real office ; they are separated from the true parietes by a ridge, which 

 evidently marks the normal outline of the valve. These intra-parietes are flat, and they 

 have a striated appearance rather difierent from the rest of the valve ; and the lines of 

 growth on them are extremely oblique, almost parallel to the inner margins of the valve. 



Scuta very slightly convex ; four-sided ; the tergal and lateral margins being divided 

 by a slightly projecting point or angle ; and this is the chief character by which the scuta 

 of this genus can be distinguished from those of Polhcipes. The umbo is seated at the up- 

 permost point, except in 8. magnum, and in 8. (?) cretce (Tab. I, fig. 1 c, and fig. 1 1 c), in 

 which species the lines of growth, instead of terminating at the angle separating the lateral 

 and tergal margins, are produced upwards, so that the valve is added to above the original 

 umbo. In B. tuberculatum (fig. 10 </), the scuta present an intermediate character between 

 that in ordinary fossil species, for instance in S.fossula (fig. 4 «), and in S. magnum and 

 cretis. The occludent margin is nearly straight, or slightly curved ; both it and the lateral 

 margin form nearly rectangles with the basal margin, which is nearly straight. Internally 

 the depression for the adductor scutorum is generally, but not always, very plain ; some- 

 times the valve is filled up and rendered solid in the upper part above the adductor 

 muscle. The apex sometimes projects freely, and is internally marked with lines of growth. 

 The internal occludent margin, or edge, is also often marked by lines of growth, and the 

 part thus marked, close above the adductor muscle, sometimes becomes suddenly wider ; 

 this is caused by some slight change in the position of the animal's body during growth. 



Terga flat, either trigonal or rhomboidal, and, in the former case, sometimes so much 

 elongated, with the carinal margin so much hollowed out, as to become almost crescent- 

 shaped ; a slight furrow often runs from the upper to the basal angle. Internally, in the 

 upper part, there is in some species a little group of small longitudinal ridges, unlike any- 

 thing I have seen in recent species, and serving, I apprehend, to give firmer attachment to 

 the corium. 



