BRACHIOPODA. 



451 



chamber of much smaller dimensions than the lateral ones " 

 (Davidson). The small central chamber must have been 

 occupied by the digestive organs, and the spiral arms must 

 have filled the great lateral spaces. In the interior of the 

 smaller or dorsal valve are two longitudinal septa, which 

 often form a chamber corresponding to and apposed to the 

 median chamber in the ventral valve. The Pentameri range 

 from the Lower Silurian to the Carboniferous inclusive ; but 

 they are especially characteristic of that portion of the Si- 



Fig. 298. Large specimen of Pentamerus oblongus, 

 Middle Silurian. (Original.) 



Fig. 299. Stricklandinia 

 Davidsoni, viewed sideways 

 and dorsally. Middle Silu- 

 rian. (After Billings.) 



lurian series known as the Llandovery formation or Middle 

 Silurian. They often occur in the greatest profusion, and 

 the species have in many cases an enormous geographical 

 range. 



In Pentamerus (as typified by such forms as P. Knightii, 

 P. galeatus, P. pseudogcdeatus, P. oblongiis, fig. 298, &c.) the 

 shell is more or less globose, the ventral valve is much the 

 largest, and the mesial septum in the same valve is very 

 long. In other allied forms which have been termed Strict- 



