SCALPELLUM. 17 
but im 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, slightly or considerably 
curved inwards, with the umbo seated at the uppermost point: S. magnum, however, 
must be excepted, for in it the umbo is sub-central, and the valve almost angularly bent, as 
will be described im 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 different 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 Pollicipes. The umbo is seated at the up- 
permost point, except in S. magnum, and in S. (?) crete (Tab. I, fig. 1 ¢, and fig. 11 ¢), 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 S. twberculatum (fig. 10 d), the scuta present an intermediate character between 
that in ordinary fossil species, for instance in S. fossu/a (fig. 4a), and in 8. magnum and 
crete. 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. 
