BRACHIOPODA. 



461 



FAM. X. LINGULID^E. Animal fixed by a muscular pe- 

 duncle passing out between the beaks of the valves. Arms 

 fleshy, not supported by calcified processes. Shell unarticu- 

 lated, sub-equivalve, of a horny texture, and partially phos- 

 phatic in composition. The Lingulidce range, in the person 

 of Lingula itself, from the Cambrian period to the present 

 day. 



In the genus Lingula (fig. 317) the shell is oblong, com- 

 pressed, the dorsal valve little smaller than the ventral. 

 The shell is oval, round- 

 ed, or satchel - shaped, 

 tapering more or less 

 towards the beaks. The 

 surface is concentrically 

 striated with lines of 

 growth. The genus 

 commences to be repre- 

 sented in the Cambrian 

 rocks, and has con- 

 tinued without inter- 

 ruption, and with no 

 perceptible change, to the present day. 



The genus or sub-genus Lingulella (fig. 316, D) differs 

 from Lingula proper in the fact that there is a distinct 

 groove for the passage of the peduncle. This type com- 

 prises the oldest of British Brachiopods, L. fcrruginea being 

 a small form which occurs low down in the Lower Cam- 

 brian. Another species viz., L. Davisii is extremely abun- 

 dant in the so-called "Lingula Flags" (Upper Cambrian). 



In Obolus (fig. 316, B) the valves are orbicular, sub- 

 equal, smooth, the ventral valve having a longitudinal 

 furrow for the passage of fibres of attachment, which pass 

 out between the beaks. 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 genus Obolella, of the Cambrian and Lower Silurian, 

 is nearly allied to Obolus, but the arrangement of the 

 muscular impressions is different. Lastly, Kutorgina (fig. 



Fig. 317. Lingula Eva. Lower Silurian. Dorsal and 

 ventral valves. (After Billings.) 



