232 PALEONTOLOGY. 



bodies by a ligament. They have two long, fleshy arms, 

 fringed with filaments, which they can protrude to a distance 

 from their body, and retract in spiral coils at pleasure. The 

 shell in which these curious arms are lodged, is lined with 

 a vascular bilobed mantle, to which the functions of shell- 

 secretion and respiration are assigned. 



Among existing genera the ligament of attachment is 

 long in Lingula, and permits the shell to float. OrHcula 

 is sessile, and adheres by one end of a short muscle which 

 perforates the valve. Terebratula is attached by a short 

 pedicle which passes through a hole in the beak-like process 

 of the valves, (fig. 162 and 163). 



The body is so placed in the shell that the back cor- 

 responds to the centre of one valve, and the abdomen 

 to the centre of the other. The shell is consequently 

 equilateral, and its valves are called dorsal and ventral. 

 Professor Owen says, that " in the terebratula the perfo- 

 rated valve must be regarded as the inferior or ventral one, 

 and the imperforate or shorter valve the dorsal one." 

 Professor Pictet holds a contrary opinion ; he says, " the 

 ventral valve is the smallest and generally supports in its 

 interior, near to the hinge, the apophysal apparatus ; the 

 dorsal valve is the largest, and, in most genera, develops a 

 beak-shaped process. The part which is bent against the 

 ventral valve is called the area; the central part marked with 

 lines of growth is the deltidium" 



The form and structure of the area and deltidium afford 

 good generic characters. The valves are frequently united by 

 a bidentate hinge, the teeth of which so perfectly inter-lock 

 in terebratula, that the valves are separated with difficulty. 

 For this reason they are rarely found asunder in a fossil 

 state. 



The palaeontological history of this class is deeply interest- 

 ing ; it has been represented in the seas of all periods, and 

 formed a fourth part of the mollusean population of the 

 primary epoch. 



In the secondary periods their numbers were relatively 

 greatly diminished, and in the tertiary seas, the brachiopoda 

 formed about one per cent, of the whole. 



This class illustrates, in a satisfactory manner, the law of 

 the speciality of fossils. 



