MARYLAND GEOLOGICAL SURVEY 95 



Description. — This species which, hecause of its great variahility and 

 wide geographical and geological distribution, has been referred to 

 under many names, was very fully described by Darwin. This descrip- 

 tion although written almost fifty years ago is more complete than 

 anything which has since been ])ublished, and little can be added to it 

 even now. 



Darwin says: " Parietes and l)asis, but not the radii, permeated by 

 pores; shells longitudinally striped with white and pink, or dull purple; 

 sometimes wholly white; scutimi finely striated longitudinally; inter- 

 nally, adductor ridge very or moderately prominent 



General Appearance. — Shell conical (fig. 4a), often steeply conical 

 (fig. -ic), but sometimes depressed and smooth (fig. 4d); orifice generally 

 rather small, varying from rhondjoidal to trigonal, with the radii nar- 

 row, and generally in the fossil specimens very oblique; surface gener- 

 ally smooth, sometimes rugged, and in the Coralline Crag specimens 

 commonly ribbed longitudinally, the ribs being narrow 



Scuta: These in young and moderately-sized specimens are striated 

 longitudinally (fig. 41), sometimes faintly, but generally plainly, causing 

 the lines of growth to be beaded; but in large and half-grown specimens, 

 the lines of growth are often extremely prominent, and being inter- 

 sected by the radiating stria?, are converted into little teeth or denticuli. 

 As the strife often run in pairs, the little teeth frequently stand in 

 pairs, or broader teeth have a little notch on their summits, bearing a 

 minute tuft of spines. In very old and large specimens, the prominent 

 lines of growth are generally simply intersected by deep and narrow 

 radiating stria (tab. I, fig. 4p). In one case, a single zone of growth 

 in one valve was quite smooth, whilst the zones above and below were 

 denticulated. The valve varies in thickness, which I think influences 

 the prominence of the lines of growth and the depth of the stria. 

 These stria? often affect the internal surface (fig. 4h) of the basal margin, 

 making it Ijluntly toothed. The articular ridge (fig. 4n) is rather small, 

 and moderately reflexed. The adductor ridge (as already stated) varies 

 remarkably; in most of the recent Panama specimens (fig. 4n), and in 

 the fossils from Portugal, it is extremely prominent, and extends down 

 to near the basal margin; in other specimens it is but slightly promt- 



