CYRTINA. 



67 



for it is evident that considerable dissimilarities in the arrangements of the plates of the 

 ventral valve must have carried along with them some important difference in the soft 

 portions of the animal, and I therefore propose at least provisionally to distinguish the 

 little groups of spiriform shells of which C. heterocli/ta, C. Demarlii, and C. se.ptosa are 

 examples under the generic or sub-generic appellation of Cyrtina,-^ and to leave that 

 of Cyrtia to those shells which agree with Dalman's C. exporrecta, C. trapezoidalis, and 

 C. Murckhoniana, &c. ; but it is necessary to observe that the last-named genus is in 

 itself of such little value that it will remain a question for further discussion whether it 

 should be retained or added to the synonyms of Sowerby's Sjnrifer. 



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, as may be perceived by a reference to the woodcut in page 45." There exists 

 in Cyrtia no median plate or septum, the arch-shaped deltidium which covers the entire 

 fissure is generally, but not always, perforated by a circular foramen.^ In the smaller 

 valve the spiral appendages and their mode of attachment is exactly similar to what we 

 find in Sjjirifer, and with which the plates in the ventral valve also very closely agree. 



n \ \ 



Penfamerus Knif/htii. 



Cyrtina heterochjta, slightly enlarged. F^S- 2- Longitudinal section. 



Fig. 3. Transversal section. 



s. Septum. V. Dental plates, x. V-sliaped cliamber. m and n. Diverging plates of the dorsal valve 

 (in Pentamevus), to which are attached the curved plates o. a. Area. d. Deltidium. 



1 The diminutive of Cyrtia is Cyrtidium, but I have preferred bad Greek to a long name. 



2 The position and extent of these dental or rostral plates may sometimes be observed on the exterior 

 of the shell, and especially on slightly worn specimens, by two diverging lines departing from the extremity 

 of the beak (PI. VII, fig. 22, s). In the upper Llandovery rock of May Hill are found many internal casts 

 of C. trapezoidalis, which exhibit in a very beautiful manner the slits produced by the diverging plates ; 

 but these may be at any time exposed by grinding a portion of the beak of the Wenlock or Gothland 

 specimens. 



3 The foramen is sometimes tubular, and especially so in certain Chinese examples of C. Murehi- 



soniana. 



