CLASSIFICATION OF THE BRACHIOPODA. 131 



articulated, and kept together by special muscles, as in Discina. Mr. Sharpe further 

 observes, that " the shell consists of layers of two distinct structures ; the outer layer is 

 punctated ; the pimctations are so large as to be clearly visible to the naked eye, and are 

 arranged quincuncially with great regularity. The inner layers of the shell are not 

 punctated, and have a fibrous and slightly striated appearance and pearly lustre ; these 

 impundate layers are thickest towards the middle of the shell, and do not quite reach the 

 margin. It thus appears that the genus Trematis differs from Orhicida {Discina) in the 

 punctated structure of the shell, and in having the valves united by a hinge, while it is 

 distinguished from Terehratula and other hinged forms of Brachiopoda, by the ligament 

 passing through the ventral valve." Having submitted a specimen of Trematis terminalis 

 to Dr. Carpenter's microscopic examination, that gentleman informs me " that it does not 

 depart in any essential particular from the type of Dischiida, the punctated surface being 

 {as in Porambonites), a mere superficial conformation; and Mr. Sharpe's internal un- 

 punctated layer being a succession of lamina, just as in Discina, and that every thing 

 now tallies satisfactorily in regard to the conformity between the general characters of the 

 shell and its intimate structure." 



In August 1847, M. d'Orbigny proposed for the same shells, the name Orbicella} 

 which must be considered as a synonym, Mr. Sharpe's paper having been published a 

 month before that of the French author ! M. d'Orbigny does not allude to a hinge or 

 diverging plates, he mentions that the structure of the shell (which he erroneously states to 

 be punctated), and the convexity of the perforated valve are the principal distinctions he 

 had observed, the animal being fixed to submarine bottoms by a pedicle similar to that of 

 Terebratula." From all the above, it seems questionable whether the shells here described 

 deserve to be erected into a distinct ye;??^*. They probably constitute simply a section 

 of Discina ; but as nothing is known of the internal impressions and characters, I shall for 

 the present admit it with reserve and doubt. 



Geol. range. — This section seems to have originated in the Silurian period, above which 

 no examples have as yet been discovered. 



Examples: T. terminalis, Emmons, sp. ; "^ cancellata, G. Sow.; "^punctata, Sow., sp. 



Genus — Siphonotretra, De Verneuil, 1845. 



Type — S. UNGUicuLATA, Eichv., sp. Int., PI. IX, figs. 261 — 268. 



Crania (unguiculata), Eichwald. 

 Teuebkatula (part), Eichw. 

 Siphonotretra, of all modern Authors. 



^ Considerations Zoologiques et Gcologiques sur les Brachiopodes ; Comptes rend us des Sciences de 

 rAcad^mie des Sciences; Paris, 5 Aout, 1847. 



2 In the An. and Mag. of Nat. Hist., vol. iv, 2d ser., p. 316, 1849. Mr. Morris seems to doubt that 

 Trematis was the same as Orbicella, D'Orb. ; a reference to the types mentioned by both authors will 

 convince any one of the correctness of my statement. 



