DAVIDSON — ON SOME KOSSIL BRACIIIOPODA, 
415 
tinguish the little group of spiriferous shells of which C. heteroclyla, C. 
Demarlii, C. scjdosa, C. dorsaia, and, perhaps, C. carhonaria, are 
examples, under the sub-generic appellation of Ctetina, and to leave that 
of Ci'ETiA to those shells which agree withC. exporreda, C. Murchisoniana' 
&c. ; 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 Cyetia 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 wo 
perceive to be the case in many species of S2)irifera. 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 
Murehisonicma. 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 Sjnrifera. 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 sufiicient importance to warrant the creation of even a separate 
£ub-genus. In the Cyktina, 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, 
a. Half of the Area. 
d. Ucltidium. 
0. and a". Dental 
idatea. 
s. Median scpium. 
Fi^. 1.--Ii0ii3itniliiial Section ol Cjitin Iict^rodjtn, 
