CLASSIFICATION OF THE BRACHIOPODA. 61 



the Cretaceous strata; it suddenly diminished in the Tertiary period, since only one 

 species, likewise now alive, has hitherto been discovered. The Family SpiRiFERiDiE, on 

 the contrary, presents its greatest development in the early periods of the habitable world, 

 and no genus or species with spiral lamellae has been as yet noticed above the Lias ; and 

 it seems as if the spiral forms had been suddenly replaced by those provided with loops, 

 which certainly implies a most important modification in the form and condition of the 

 labial appendages ; and it seems much more natural, and I may say proper, to place in 

 one family all the species provided with loops, and into another all those possessed of 

 spires. The same may be stated as regards the Strophomenida and Productida, no 

 example of which has been found higher up than the Lias. 



Viewed, therefore, in a general way, the number of sections, according to our present 

 information, would be distributed somewhat as follows : 



In the Silurian 23, in the Devonian 23, in the Carboniferous ] 6, in the Permian 

 13, in the Triasic 8 or 9, in the Oolitic 11, in the Cretaceous 14, in the Tertiary 10, 

 in the present period 14 : or, in other words, 33 genera lived during the Palceozoic epochs, 

 or up to Trias included ; from thence upwards to the present period, the number seems 

 to have been reduced to about 20, of which 8 only are found in both the two great 

 divisions ; but, as above observed, every day may bring fresh changes in our statistics, and 

 the views here expressed can only be looked upon as approximative, besides which a few 

 of the sections, such as Trigonosemus, Terebrirostra, Spiriferina, Ci/rtia, Betzia, Aulosteges, 

 Ofbiculoidea, Trematis, Acrotreta, and perhaps one or two others, require further exami- 

 nation before being finally admitted. 



We will now proceed to describe each genus in detail, as far as our present infor- 

 mation will admit. 



Family— TEREBRATULID^. 



Animal fixed to submarine bottoms by a muscular peduncle issuing from a perforation 

 in the beak of the larger valve ; this aperture is partly surrounded by a deltidium in 

 one or two pieces : oral appendages entirely or partially supported by calcified processes, 

 which commonly assume the shape of a loop, variable in form and dimensions, but always 

 fixed to the smaller or dorsal valve : shell structure always punctated. 



Obs. In this natural family we have admitted seven genera and five sub-genera or 

 sections ; with the exception of two of these last, viz. Terebrirostra and Trigonosemus, 

 which are still problematical, all other genera and sections are based on well-defined 

 modifications in the form and position of the calcified supports of the arms. 



