BRACHIOPODA. 85 



BRAC1II0P0DA. Lam. A family of symmetrical bivalves be- 

 longing to the third section of Lamarck's order " Conchifera 

 Monomyaria," described as bivalve (generally symmetrical) ad- 

 hering to marine bodies, by a tendon passing through the shell, 

 having no true ligament. What most distinguishes this family and 

 renders it remarkable is the structure of the animal. It has two 

 elongated, tendril-shaped arms. When the animal is in a state of 

 repose these arms are coiled up spirally and enclosed in the shell, 

 but when required for use, are unfolded and extended. This 

 family contains the genera Orbicula, Terebratula and Lingula, in 

 the system of Lamarck, to which may be added Thecidium, Pro- 

 ductus, Spirifer, Magas, Pentamerus, Crania, Strigocephalus, 

 Strophomena, and some others enumerated in the explanation 

 of figures 201 to 219. The above genera may be thus dis- 

 tinguished. 



1. Orbicula. Umbones central ; byssus passing through a 



hole in the flat valve. Fig. 201. 



2. Atrypa. Without foramen or space between the valves. 

 Fig. 203. 



3. Producta. The same, valves produced, overwrapping ; 



including Leptsena. Fig. 206, 206*. 



4. Terebratula. Hinge of the upper valve produced be- 



yond that of the other, with a pit or foramen ; including 

 Delthyris, Orthis, Trigonosemus, Magas, Strophomena. Fig. 

 202, 205, 207, 208, 209. 



5. Spirifer. The same, with deep triangular area ; spiral 

 folds in the interior ; including Trigonotreta and Cyrtia. 

 Fig. 204, 214, 215. 



6. Thecidium. Large valve attached ; curved ridges in 



the inner surface ; two jutting points or teeth on the hinge. 

 Fig. 216. 



7. Crania. Attached by the surface of the valve ; muscular 

 impressions four, forming a face. Fig. 197, «, b. 



8. Pycnodonta. Irregular; hinge with raised pointed teeth. 



Fig. 217, 218. 



