BRACHIOPODA. 



6 5 I 



and the genera Neobolus, Lakhmina, and Schizopholis being Car- 

 boniferous. 



In the genus Obolus (fig. 495, b and c) the shell is orbicular, 

 with subequal, smooth valves, the ventral valve having a longitu- 



Fig. 495. — Li7igulidce, Discinidce, and Obolidce. a, Trematis Jilosa, Ordovician ; b, Cast of 

 Obolus Davidsoni, Silurian ; c, Interior of the dorsal valve of the same; d, Lingzilella Davisii, 

 Upper Cambrian ; e, Ventral valve of Acrotreta Nicholson, of the natural size and enlarged, 

 and f, Side-view of the ventral valve of the same, enlarged, Ordovician ; g, Interior of the dorsal 

 valve of the same. (A is after Billings, and the other figures after Davidson.) 



dinal groove for the passage of the peduncle-fibres. Usually there 

 is a weak median septum in the ventral valve. The species of this 

 genus are wholly Ordovician and Silurian, and are especially char- 

 acteristic of the former of these systems. The little Obolus Apolfaiis 



Fig. 496. — Kutorgina cingulata. (Billings.) Upper Cambrian. 



occurs in vast numbers in the Ordovician " Ungulite Grit " of 

 Russia. 



The genus Obolella, of the Cambrian and Ordovician formations, is 

 nearly allied to Obolus, but the arrangement of the muscular impressions 

 is different. The genus Kutorgina (fig. 496) is essentially similar to the 

 preceding, but the shell is sub-quadrate, and there is a straight hinge-line. 



