BRACHIOPODA 51 



internal loop, and a very prominent cardinal process, forked 

 at the end, and fitting over the central plate of the opposite 

 valve. 



The shell of Tcrcbratula and some of its allies (Argiopc, 

 Thecidium, Gyrtia, and Spiriferinaj is dotted with minute 

 quincuncial perforations, sometimes visible to the naked eye, 

 as in T. lima, but usually requiring a lens of low power. 

 They are smallest in T. carnea. 



The lamp-shells with sharp beaks and plaited valves have 

 been separated from the Tcrebratulce under the name Rhyn- 

 chonclla (Fisch.) Their shells do not exhibit the punctate 

 structure under a magnifying-glass, and they have no internal 

 skeleton to support their arms, which in the recent species are 

 coiled up spirally, and directed towards the concavity of the 

 smaller valve, bike the spires of the extinct Atrypa (fig. 11, 7). 

 Of the three living species of Rhynehonclla, one is found 

 throughout the Arctic Seas, a second in New Zealand, and the 

 third at the Feejees (V). The fossil species exceed 250, and 

 are found in all parts of the world ; those from the palaeozoic 

 strata may prove distinct from the rest, since the permian 

 species are known to be provided with large internal processes 

 {Camarophoria, King). Casts of these shells are frequently 

 impressed with the narrow and angular pallio-vascular impres- 

 sions. The extinct genus Atrypa differs from Rhynehonclla 

 solely in 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 Rhynehonclla. 

 The lower Silurian rock contains another genus, Poranibonitcs 

 (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 



