BRACHIOPODA. 57 



solely ill having calcareous spires, which are preserved in 

 many instances, and may be cleared to some extent by the 

 application of acid. The foramen is separated from the hinge- 

 line by a deltidium; and the interior of the valve is marked 

 by ovarian and vascular spaces exactly as in Bhynclwnella. 

 The lower Silurian rock contains another genus, Poramhonites 

 (Pander), as yet imperfectly understood, but having the valves 

 marked externally by impressed dots, which are not perfora- 

 tions. The genus Pentamerus occurs in all the strata below 

 the carboniferous limestone, and is remarkable for its great 

 internal partitions, causing the shell to split readily across the 

 middle ; and giving rise to deep incisions in those casts of 

 the interior which are so common in the Caradoc sandstone 



(fig. 12, 8). 



The extinct Spiriferidce are a family characterized by the 

 possession of internal calcareous spires extending from the 

 centre of the shell outwards (fig. 12, 3). These spires, like the 

 shell itself, are frequently silicified, and may be disengaged 

 from the matrix by the action of acid. At other times the 

 shell is imbedded in soft marl, removable by careful washing, 

 so as to shew the calcareous lamina of the spire fringed with 

 hair-like processes, formerly the support of cirri. In the 

 genus Spirifera the shell has a long straight hinge-line, and the 

 flattened area of the larger valve has a deltoid byssal notch.* 

 The typical species are characteristic of the palaeozoic strata, 

 and have a shell-structure like Rhynchonella. The liassie 

 species (Spiriferina, d'Orb.), have punctate shells, and the 

 byssal opening is closed (at least in the adult) by a thin 

 arched plate or "pseudo-deltidium." In the sub-genus Cyrtia 

 (fig. 12, 4), the hinge-area is ultimately as long as it is wide, 

 and the deltidium is perforated in the centre by a byssal 



* The term deltidium, applied by Von Buch to this foramen, has, by miscon- 

 ception of his meaning, become constantly used for the plates which partially 

 close it. 



