OCR Text |
Show 1866.] MR. T. DAVIDSON ON JAMAICAN BRACHIOPODA. 103 2. ARGIOPE BARRETTIANA, Dav. (PI. XII. fig. 3.) Shell small, somewhat pentagonal; dorsal valve semicircular, slightly indented in front; hinge-line straight, as long as the entire breadth of the shell, and forming acute angles at its junction with the lateral margins of the valves ; hinge-area narrow; valves unequally convex, dorsal valve very slightly so and with a shallow depression along the middle; ventral valve very convex, with a deep sinus. Surface ornamented with from eight to twelve corresponding radiating rounded ribs, with interspaces of almost equal width, but of which one is a little wider and deeper along the middle of each valve, while in the sinus of the larger valve there sometimes exists a smaller or shorter rib. Area in the ventral valve acutely triangular, foramen large, deltidium rudimentary : shell punctate. In the interior of the dorsal valve the apophysary system consists of a distinct loop or riband-shaped lamella, originating at the base of the sockets, and forming a curve on each side so as to meet and adhere to the single central prominent submarginal septum, this lamella being also partly confluent with the valve. Proportions variable ; the largest specimen measured about 3 lines in length by a little more than 4 in width. This beautiful Argiope is of a light yellow colour, the interspaces between the ribs being slightly tinged with red. It approaches most nearly to Argiope cuneata (Risso), but is a much larger shell, and possesses when full-grown a larger number of ribs. In external shape it also bears some resemblance to certain examples of A. de-collata, as well as to some other fossil species, but may be at once distinguished by its loop, which is two- and not four-lobed. Hab. North-east coast of Jamaica, at a depth of about 150 fathoms. I have named this shell after m y lamented friend Lucas Barrett, whose life was so prematurely sacrificed to his zeal for scientific discovery. 3. XARGIOPE WOODWARDIANA, Dav. (PI. XII. fig. 4.) Shell very small, somewhat pentagonal, indented in front; dorsal valve semicircular; hinge-line as long as the width of the shell, moderately convex, but divided into two lobes by a deep median sulcus ; ventral valve deeper and more convex than the opposite one, and with a longitudinal groove along the middle ; beak very prominent ; area acutely triangular, foramen large, deltidium rudimentary. External surface smooth, whitish yellow, with a few red patches arranged in radiating interrupted lines from the beaks. The shell is also marked with numerous concentric lines of growth. In the interior of the dorsal valve the loop is two-lobed, adhering to a central submarginal elevated septum. Shell-structure punctate. Proportions variable; the largest specimen measured, length 2^, breadth 2, depth If lines. This interesting stout little shell differs from its congeners in its shape as well as in its smooth and spotted surface. It was also found alive by the late Lucas Barrett, at a depth of 60 fathoms, on the north-east coast of Jamaica, and is a much smaller shell than |