130 INTRODUCTION. 



fixed to submarine bottoms by a pedicle, which afforded greater freedom of motion to the 

 animal than that of Discina. About the same period, Professor S. Kutorga likewise 

 separated the above-mentioned shell from the Lamarckian genus, by the name of 

 Scldzotreta} bringing forward similar reasons to those urged by the author of the 

 ' Palaeontologie Fran^aise ;' but it is still very doubtful, if the section ought to be 

 admitted. 



Geoh range. — According to M. d'Orbigny, Orbiculoidea first appeared in the Silurian 

 epoch, more numerous during the Carboniferous period, and disappeared in the Neocomian 



age. 



Examples : 0. elliptica, Kutorga, &c. 



Section B. Trematis, Sharpe, 1847. 



Type — T. terminalis, Emmons, sp. Int., PI. IX, figs. 256 — 260. 



Orbicula, of the generality of Authors. 

 Trematis, Sharpe (June, 1847). 

 Orbicella, D'Orbigny (August, 1847). 



Animal unknown ; shell somewhat depressed, sub-orbicular or transversely oval ; both 

 valves slightly and unequally convex, giving a lenticular form to the shell ; umbo of the 

 upper or imperforated valve submarginal, slightly projecting. Lower or ventral valve with 

 a sub-central umbo, behind which a narrow, oblong, oval slit reaches to near the posterior 

 margin, and afforded passage to the pedicle fibres of attachment. Internal dispositions 

 unknown, a wide cardinal margin is visible in the imperforated valve. 



Obs. Mr. Sharpe proposed to separate from Biscina certain shells which in their 

 general external appearance bore much resemblance to that genus, but appeared to differ 

 by others, which he thus defined :" " The valves are united hy a hinge, of which the details 

 cannot be seen in the specimens; hut it is probably formed of two diverging lamellar pro- 

 cesses in the dorsal valve, for where the shell of that valve has been worn away we can 

 trace three calcareous plates diverging from the hinge of the dorsal valve, as in the Leptse- 

 noid species of Orthis, and in many of the Spirifers. Wherever these plates are found in the 

 Brachiopoda, the outer pair appear to be continuations of the teeth or lamellar processes of 

 the hinge ; so that the presence of such plates is sufficient to show that the valves played 

 upon a hinge. The third or mesial plate separates the two great adductor muscles." 

 The above statements require, however, further examination before being admitted, nor 

 have I been able to obtain examples adding much light to the subject ; but I am far 

 from being convinced that Trematis was provided with an articulated hinge resembling 

 that of Terebratula ; on the contrary, I am inclined to believe the valves were un- 



1 Ueber dei Siphonotretaeae aus den Verhandlungen der Kaiserlichen Min. Ges. fiir das Jahr 1847. 

 ^ Quart. Journal of the Geol. Soc, No. 13, vol. iv, p. 66. 



