96 SYSTEMATIC PALEONTOLOGY 



nent, as in those from the Crag (4f'); it is short, but rather prominent 

 in the specimens (4h) from Maryland; whereas it is very slightly promi- 

 nent in the specimens from Virginia. The cavity for the lateral 

 depressor, also, varies greatly; it is often, as in recent specimens, 

 bounded on the side towards the occhident margin by a very slight 

 straight ridge, which occasionally folds a little over, making almost a 

 tube; this, at first, I thought an excellent specific character, but far 

 from this being the case, the cavity often becomes, in recent specimens 

 as well as in the Crag specimens (4f), wide, quite open, and shallow. 

 The whole valve in the Crag specimens (fig. 4e) is apt to be more elon- 

 gated than in the recent or Portuguese specimens (fig. 41), and especially 

 than in the Maryland (fig. 4h) specimens. 



Terga very slightly beaked; the surface towards the carinal end of 

 the valve, in some of the fossil specimens, is feebly striated longitudi- 

 nally. There is either a slight depression (fig. 4k), or more commonly 

 a deep longitudinal furrow (fig. 4g, 4o) with the edges folded in and 

 touching each other, extending down the valve to the spur, and causing 

 the latter to vary in width relatively to its length. When the furrow 

 is closed in, the spur is about one-fourth of the entire width of the 

 valve, and has its lower end obliquely rounded, and stands at about 

 its own width from the basi-scutal angle: when there is only a slight 

 depression and no furrow (as is the case with young specimens, and 

 in the specimens (4k) from Maryland), the spur is broader, equalling 

 one-third of the width of the valve, with its lower end almost truncated, 

 and standing at about half its own width from the basi-scutal angle. 

 But the absolute length of the spur also varies considerably in the 

 Coralline Crag specimens; it is often very long, (fig. 4g) compared to 

 the whole valve. In many Italian specimens (4o) it is long and broad. 

 The basal margin of the valve on the carinal side of the spur is some- 

 times slightly hollowed out; and when the longitudinal furrow is closed, 

 this side slopes considerably towards the spur. Internally, the articular 

 ridge and the crests for the tergal depressor muscles are moderately 

 prominent. 



Parietes. — The longitudinal septa sometimes stand near each other, 

 making the parietal pores small. The radii have oblique summits, but 



