BRACHIOPODA. 



213 



concentric lines of growth, and furnished with numerous deli- 

 cate tubular spines, which, however, are rarely preserved. All 

 the Siphonotreta at present known belong to the Silurian period. 



FAM. X. LINGULID^E. Animal fixed by a muscular peduncle 

 passing out between the beaks of the valves. Arms fleshy, not 

 supported by calcified processes. Shell unarticulated, sub-equi- 

 valve, of a horny texture. The Lingulidce range, in the person 

 of Lingula itself, from the Cambrian period to the present day. 



In the genus Lingula, (fig. 165) the shell is oblong, com- 

 pressed, the dorsal valve little smaller than the ventral. The 



Fig. 165. Lingula Eva, Lower Silurian. Dorsal and ventral valves. 

 (After Billings.) 



shell is oval, rounded, or satchel-shaped, tapering more or less 

 towards the beaks. The surface is concentrically striated with 

 lines of growth. The genus commences to be represented in 

 the Cambrian Rocks, and has continued without interruption, 

 and with no perceptible change, to the present day. 



In Ob0lus the valves are orbicular, sub-equal, smooth, the 

 ventral valve having a longitudinal furrow for the passage of 



Fig. -L^.Obolella cingulata, Billings. Upper Cambrian. 



fibres of attachment. The valves are unarticulated, and are 

 maintained in apposition by muscular action. All the known 

 species of Obolus are confined to the Silurian period, and are 

 especially characteristic of the Lower Silurian period. The 



