BRACHIOPODA. 



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., 

 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 hracMal or 

 dorsal valve. The peduncle valve, which is usually the larger and 

 uppermost, contains the bulk of the viscera ; in the higher genera, 



Fio-. 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, b, 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 



