BRAOHIOPODA. 
75 
stitute only a small remnant of a group that flourished abundantly 
in former epochs. There are about 150 recent, and over 6000 
fossil species. 
The Shell. — The valves of a Brachiopod shell differ from each 
other in size and shape, but each valve is in itself symmetrical, i.e. y 
similar on each side of a middle line. 
The valve through which the peduncle passes is termed the 
•peduncle or ventral valve (Fig. 2, A), the other being the brachial or 
dorsal valve. The peduncle valve, which is usually the larger and 
uppermost, contains the bulk of the viscera ; in the higher genera,. 
Fig. 2. 
Magellania flavescens. Australia. Interior of valves. 
A. Peduncle valve : /, foramen for peduncle, below which are the' two small 
deltidial plates ; t, hinge teeth ; a, o, c, muscle scars. B. Brachial valve, 
showing the reflected loop for support of the " arms." 
calcareous bars or loops (Fig. 2, B) attached to the inner surface of 
the brachial valve form a support for the "arms" of the animal. 
The inner surface of the valves presents certain markings and 
depressions where the muscles have been attached (Fig. 6). 
The shell is constructed of very minute prisms of calcareous 
substance imbedded in an organic matrix. In Lingula the shell is 
formed of alternating layers of horny and calcareous substance. 
The shell-valves are either hingeless, or joined by a hinge in 
which teeth in the peduncle valve fit into sockets in the brachial 
valve. The Brachiopoda are primarily divided into two sections, 
Inarticulata and Articulata, based on the absence or presence of a 
hinge. 
The division into Orders is based on the relation of the peduncle 
