256 BRITISH PALAOZOIC FOSSILS. [Bracuiopopa. 
margin; beneath the beak the shell is greatly thickened, forming an obtuse wide pad, internally reaching nearly 
to the margins: beak of larger valve terminal, slightly produced, apparently channelled below by a triangular 
groove, on each side of the base of which is a strong conical boss, projecting into the cavity of the opposite 
valve, like cardinal teeth ; surface dull, nearly smooth, or shewing under very strong glasses an indistinct, very 
minute punctation (perhaps due to the fibrous tissue of the shell) and delicate concentric strie. Length four 
lines, width 41 lines. 
The specimens I have examined are chiefly internal impressions, shewing two very deep, small, oval pits 
near the hinge-line, resembling the so-called Crania Sedgwickii of Davidson (Bull. Géol. Soc. de France for 
October, 1848), from which it differs in the greater approximation of these impressions (which are clearly not 
analogous to the muscular impressions of Crania), smaller size, We. 
Position and Locality —Five or six specimens have occurred close together in the shale of Builth Bridge, 
Radnorshire. 
Explanation of Figures.—P\. 1. H. fig. 4. Group of three casts of inner surface of large valve, natural 
size ; fig. 4a, wax cast from one of ditto, shewing the appearance of the interior of the shell, magnified; fig. 5, 
exterior of small specimen of small valve; fig. 5 a, interior of ditto, shewing the thickening of the shell beneath 
the apex ; 56, portion of external surface more highly magnified. 
3rd Class. LAMELLIBRANCHIATA. 
Turs class has the two shelly valves placed at right angles to their position in the Palliobranchiata, one 
valve being applied to each side; they are connected by a ligament, within which is an elastic substance termed 
cartilage, which forces the valves open; the valves are closed in the lower groups by one central adductor muscle, 
and in the higher types by two or more. Respiration (as the name implies) is effected by lamellar branchize, 
or gills, which are four, crescent-shaped, and attached to the inside of the mantle; one pair on each side of 
the visceral mass. Each valve is lined by a lobe of the mantle, and the attachments of the muscular fibres for 
retracting its edges form the pallial scar in the shell; in the low types the lobes are open and disconnected, 
as in the Palliobranchs, and there is little or no foot; in the higher types the edges of the two lobes are united 
posteriorly, and produced into two tubular siphons ; one branchial, through which the water enters to the gills, 
and carrying nutrient particles to the mouth; the other anal, through which the impure water constantly flows 
out, the currents in each case produced by vibratile cilize ; the attachments of the line of muscular fibres for 
retracting the siphon form the “ pallial sinus ” in the shell of such as have the siphons. The animal is without 
head or cephalic organs of sense ; the mouth is at the anterior end, without organs of mastication, having two 
strap-shaped, sensitive tentacles on each side, the upper pair of which correspond, on a reduced scale, with the 
spiral arms of the Brachiopoda ; the lower pair (which equal the upper in size) are analogous to the transverse, 
brachial, fringed part below the mouth in the same; a short cesophagus leads into a pear-shaped stomach, 
leading to a convoluted intestine, the straight terminal portion, or rectum, perforating the heart and terminating 
at the base of the anal siphon, or over the posterior adductor. ‘The Jiver is a large follicular mass, enyeloping 
the stomach; a glandular mass, partially surrounding the rectum, is called Aidney, by Owen, from uric acid 
having been found in it, but is marked ovary by Milne Edwards (both agree that the ovary in the female, 
and testis in the male, surrounds the intestine). The foot, when it exists, is a symmetrical, fleshy, extensile 
organ, developed from the ventral aspect of the body ; the enveloping muscular fibres are transverse, the 
action of which protrudes the foot, and longitudinal, the action of which retracts it. In the lower bivalves, 
(Monomyaria,) the heart is of a single auricle, and one ventricle, perforated by the rectum; in the higher 
groups (Dimyaria) the veins from the gills open into two auricles, which open into one fusiform ventricle, 
perforated by the intestine (in Avca the ventricle is double). An artery from each end of the ventricle 
