B. VARIANS. _G. B. Sowerby_, in Darwin's Geolog. Observ. on South
America, (Sept. 1846), Tab. 2, fig. 4, 5, 6.
_Parietes moderately thick: radii with their upper margins very
oblique; sutural edges almost smooth, or finely crenated: basis finely
porose. Tergum with the spur small, narrow, bluntly pointed._
_Hab._--Port St. Julians, Patagonia; ancient Tertiary formation.
Eastern plain of Tierra del Fuego (?).
This species comes so close to _B. unguiformis_, that I have some
doubt whether they ought to be specifically separated: the whole shell
is stronger, and the basis can be seen to be porose when a polished
section is made: the spur of the tergum is smaller, more pointed and
more medial, but these latter differences may be due to mere variation.
Should _B. varians_ and _unguiformis_ prove to be the same species, the
latter name has the priority.
_General Appearance._--Shell moderately strong and thick; shape
conical or tubular, or even inverted conical; orifice moderately
toothed, large, sub-trigonal; walls either smooth or longitudinally
folded; the elongated specimens are most apt to be smooth. The
Radii are narrow and oblique. Diameter of largest specimen above
three-quarters of an inch.
_Scuta_, with the lines of growth moderately prominent; the internal
surface of the valve has been ill preserved; but a very prominent,
hardly reflexed, articular ridge, can be distinguished, as well
as the absence of an adductor ridge. _Terga_, with no distinct
longitudinal furrow running down the valve: spur short, bluntly
pointed, narrow, about one fifth or one sixth of width of valve;
placed at above its own with from the basi-scutal angle; the
basal margin, on each side close to the spur, curves towards it.
Internally, all that can be distinguished, is that the articular
ridge is prominent.
_Parietes_; their inner surfaces appear to have been nearly smooth;
the absence of parietal pores could be made out only by polishing
a transverse section. The Radii are narrow, and have their upper
margins very oblique and rather smooth: in the elongated varieties
the sutural edge appears to be almost absolutely smooth; in the
conical specimens it is slightly crenated, the septa being apparently
not denticulated. In living species we have a similar variation in
the state of the sutural edges of the radii, in _B. balanoides_ and
_crenatus_ the edges being much smoother in much elongated specimens
than in other varieties. The _alæ_ have their upper margins less
oblique than those of the radii, with their sutural edges barely
crenated. The _basis_ is either flat, or, in the elongated specimens,
deeply cup-formed; in section it can be seen to be finely and
irregularly porose.