Pvn H,i I'UitlS

18 TEREBRATULID.E.
A. Shell ribbed.
1. Argiope decolla'ta*, Chemnitz.
Anomia decollate,, Chemn. Conch. Cab. viii. p.96, pi. 78. i.lOba-d. Argiope
decollate,, Jeffr. in Ann. Nat. Hist. 3rd ser. ii. p. 124, pi.
v. f. 3 a-e.
Body reddish-brown : mantle forming a thin film : arms
nearly circular, divided into lobes, which correspond in number
with the septa in the shell ; cirri few and thick : 'peduncle very
short.
Shell of an irregular shape, varying from round to trans-
versely oval, sometimes resembling that of a horse's hoof,
compressed, much higher near the beak, and sloping abruptly
in a wedge-like fashion towards the front and sides, solid, of
a dull : aspect sculpture, 15-20 slight ribs which radiate from
the beak but scarcely reach the margins ; lines of growth
rather strong or conspicuous ; tubercles large and numerous,
the interstices having a frosted appearance arising from a
minute granular structure : colour light-brown : margins
rounded at the front and sides, so as to form a semicircle, and
obtusely angled behind : bealc rather prominent and worn by
continual friction ;
the under side shows distinctly the layers
of increase : foramen
exceedingly large, transversely
and irre-
gularly oval, in consequence of its having been made partly
out of the umbo of the lower valve : deltidium scarcely traceable
: hinge-plate remarkably thick and broad : teeth short and
triangular: sockets broad and deep: skeleton composed
of a series
of narrow riband-like plates, which are curved and fit into the
hollows between the septa, lying at no great distance from the
front margin, and almost touching the shell ; within the upper
valve also are five septa, placed at equal distances, the middle
and longest of which springs from under the beak ; none of
them reach the margin, which is wedge-shaped ; in the lower
valve are three strong equidistant ridges, which are placed in
the central space ; these are notched in front, crested at the
top, and obliquely striate at the sides, where may be occasionally
observed a few small bead-like tubercles. L. 0*3.
B. 0-285.
Habitat :
18 fathoms, in gravelly shell-sand, two miles
east of Guernsey ; rare. This is the most northern limit
* Truncate.