DAVIDSON — ON SOME FOSSIL BRACHIOPODA. 



415 



tinguish the little group of spiriferous shells of which C. heteroclyta, C. 

 Demarlii, C. se^itosa, C. dorsaia, and, perhaps, C. carhonwia, are 

 examples, under the sub-generic appellation of Cyetina, and to leave that 

 of Cyrtia to those shells which agree withC. exporreda, €. Murchisoniana* 

 kc. ; but it is also necessary to observe that Dalman's genus is in 

 itself founded upon a character of such little value that it will remain 

 a question for further discussion whether the name should be retained, 

 or be added to the synonyms of Sowerby's Spirifera. In Cyrtia a 

 short hinge-tooth is situated on either side at the base of the fissure, 

 supported by vertical shelly plates which diverge and extend from the 

 extremity of the beak, forming the fissure- walls, and occupying about 

 one-third of the length of the bottom of the valve, exactly as we 

 perceive to be the case in many species of Spirifera. There exists in 

 Cyrtia no median plate or septum, nor does any occur in Spirifera, 

 but the arched-shaped deltidium which covers the entire fissure is 

 generally, although not always, perforated by a circular foramen which 

 is sometimes even tubular, as in certain Chinese examples of Cyrtia 

 j\Iurcliiso7iiana. In the smaller valve, the spiral appendages are directed 

 outwards, nearly filling the cavity of the shell ; and their mode of 

 attachment is exactly similar to what we find in Spirifera, Therefore, 

 Cyrtia presents no other features by which it can be separated from 

 Spirifera than those of its deltidium and foramen, which are characters of 

 hardly sufficient importance to warrant the creation of even a separate 

 sub-genus. In the Cyrtina, however, the diverging plates above described 

 do not exist ; but we find in the interior of the ventral valve, two conti- 

 guous vertical septa which coalesce into one median plate, extending from 

 the extremity of the beak to within a short distance of the frontal margin, 



ft. Ilaifofthe Area* 



(h Ueltidlnm. 



V. and X. Dental 



plates, 

 s. Median sopium. 



Ficf. l.—Lorxjitudiual Section of C^riui heteroclyta. 



