186 BRITISH PALAXOZOIC FOSSILS. [ Bracnioropa. 
The mantle also deposits the shell, which is either, Ist, horny and fibrous, as in Lingula and Orbicula, in 
which the mantle is not attached, the mouth has two, free, spiral extensile appendages, and the margin of 
the mantle is provided with very long corneous fringes ; 2nd, testaceous and fibrous (as in the recent Hemithyris 
P3ittacea), having also apparently the mantle with thin undeveloped edges, and unconnected with the shell, 
and fleshy, free, spiral arms, supported on simple apophyses (all the fossils with similar disconnected apo- 
physes as Rhynchonella, Strigocephalus, Pentamerus, &c., have dense fibrous shells and are supposed to have 
had similar soft parts, also some without apophyses, as Orthis, &c., and in those with spiral apophyses, 
except Spiriferina) ; 3rd, testaceous punctured shells (recent Terebratula, Megathyris, &c.), having the mantle 
closely adherent to the shell, and giving off processes which project through all the minute perforations, so 
that the shell is an integrant part of the animal as in the Bryozoa. ‘This latter structure is found in all 
genera with loop-shaped connected apophyses, in one with spiral apophyses (Spiriferina), and in the 
thick-shelled, armless udistes, under the form of branching tubular canals. This punctured shell, therefore, 
co-exists with no arms (/twdistes), or with arms having little or no power of motion, and D’Orbigny 
suggests that the connexion kept up between the respiratory mantle and the exterior through these pores 
is a compensation, enabling the animal to breathe even with the shell closed. The spines of Siphonotreta and 
Producta are of similar nature, the site of each moveable fimbria of the mantle interrupting the Sojgaswoe 
of earthy matter, leaves a puncture or channel in the shell. 
The class contains two Orders. Ist Rudistes; 2nd Brachiopoda. 
Ist Ord. Rupisres (Lam.) 
Animal without arms; borders of the mantle greatly developed, lobed, and ciliated, representing the 
arms of the Brachiopoda in function; shell very thick, rarely symmetrical. 
The order contains three families :—Ist,. Thecide ; 2nd, Caprinide; 3rd, Radiolide ; not found in the 
Paleozoic rocks. 
2nd Ord. BRAcHIOPODA. 
This order contains a large group of symmetrical, usually thin, sharp-edged shells. The border of the 
mantle is little developed, and the group derives its name from two long arms, one on each side of the mouth, 
which may be regarded as an enormous development of the labial palpi of Lamellibranchiata, among which 
Anomia alone shews anything approaching the same development. These arms are hollow tubes, coiled or 
doubled up, and capable of a greater or less protrusion by the contraction of a doubly oblique set of muscular 
fibres, with which they are coated, acting on the fluid contained within them; they unite in front of the mouth, 
and have their outer margin fringed with filaments, the motion of which, whether protruded beyond the 
shell or not, is supposed to produce a strong current of water, conveying nutrient particles, towards the 
mouth; the absence in them of large blood-vessels shews they are not respiratory. The edge of the mantle 
is also fringed with long fleshy, or semicorneus filaments, covered like the surface with microscopic ciliz, 
and exciting currents of pure water over the surface for respiration, the blood being exposed to it in four 
thick branching pallial vessels in the lower, and two similar in the small or upper valve; these great 
vessels communicate with a vascular sinus, ventricle, or heart, on each side of the body, and from the 
opposite ends of those hearts come the vessels supplying the viscera. Digestion: the mouth is in the middle 
close to the base of the arms, leading (in Terebratula) by a short cesophagus into a wide stomach, sur- 
rounded by a liver or group of fepane follicles ; the stomach bending a little, ends in a short straight 
intestine (convoluted in Lingula), terminating on the right side. Nervous system: a nervous collar, with 
small ganglionic swellings, surrounds the cesophagus, and snes off twigs to the mantle and the adductors. 
The order has been divided into the following families:—lst, Craniadw ; 2nd, Orbiculide ; 3rd, Tere- 
bratulide ; 4th, Magaside; 5th, Spiriferide ; 6th, Uncitide ; 7th, Rhynchonellide ; 8th, Orthiside ; 9th, 
Productide ; 10th, Caleceolide ; 11th, Lingulide. 
