no A POPULAR EXPOSITION OF THE 



have nearly equilateral sheila. A depression or "sinus" frequently 

 occurs down the centre of one valve, and a corresponding projection 

 or " mesial fold" down the centre of the other. The sinus is almost 

 invariably on the ventral, and the fold on the dorsal valve. The 

 pointed upper extremity of the valve, is tech- 

 nically known as the " beak." In some forms 

 the valves are close together : but in others, 

 a closed space (often striated across) occurs 

 between the two. This is called the " area." 

 See Fig. 88 and accompanying explanation. 

 In the centre of the area, or under the beak ^__^^ 



of the ventral valve, there is frequently (as /! \ 



in the spirifers, &c.,) a triangular or circular |f> "V j 



orifice, the *' foramen." This opening, in the y ) y 



species which possessed it, served for the ^ 



passage of the pedicel by which the animal ^' ' 



was attached to the sea-bottom. The foramen is situated, at other 

 times, upon, or near to, the ventral beak, as in spirigera, &c. In 

 many species again, it appears to have become closed by age ; and in 

 others, it is altogether absent. The line of junction between the upper 

 part of the valves is termed the hinge-line. It is straight in some 

 genera, {Orthis, Strophomena, Spirifer^ for example,) and arched or 

 curved in others, {Athyris, Rhynconella, Pentamerus, Terebratula, etc.) 

 In many brachiopods, the shell is traversed by minute pores or tubu- 

 lar prolongations. "When this is the case, the shell is said to be 

 " punctate ;" 'and when the pores are absent, it is termed " im- 

 punctate." 



The brachiopods possess, as their chief characteristic, a pair of long 

 fleshy " arms," covered with delicate cilia, and either entirely confined 

 in a coil within the shell, or capable of protrusion to a certain extent. 

 In some genera, the inside of the dorsal valve carries peculiar spiral 

 processes, or a shelly loop or other calcareous framework, for h e 

 support of these arms. A support of this kind is however wanting 

 in many genera, or is otherwise merely rudimentary. The brachio- 

 pods differ essentially from the lammellibranchiate bivalves in the 

 non-possession of distinct branchiae or breathing gills. In existing seas 

 the brachiopods are comparatively rare, the number of known species 



• 2>=dorsal valve. F=ventral valve, a, area; 6, beak of ventral valve ;/, forameu ;* -*, 

 the hinge line ; m, position of oiesial fold ; a, position of mesial sinus. 



