ANIMALS. 127 



is present in Trigonotreta cyrtana,^ and T. attenuata^ which, from their punctures being 

 so small as to be with difficulty distinguished, would doubtlessly be placed among the 

 non-punctated Spiriferida . 



After a careful consideration of the present subject, I am led to believe that it will 

 be the most advantageous plan to group all those Spiriferida in Trig onotr eta ha^^ng 

 an area, regardless of its width ; the valves ribbed or striated ; and the large valve 

 furnished with dental plates, and occasionally a median plate, irrespective of their size. 



Choristites Mosquensis, Waldh., and Spirifer undulatus, J. de C. Sow., according to 

 this view, are generically inseparable ; as the long dental plates of the first are merely 

 enlargements of their rudimentary homologues in the last : Spirifer princeps, M'Coy, 

 and Brachytltyris planicostata, M'Coy, are also referable to one and the same genus ; 

 the one having simply a greater lateral development than the other : while Spirifer 

 Walcotti, and Delthyris cyrtana,^ are equally unseverable ; because the large median 

 plate of the former is more developed than its homologue of the latter. 



The foregoing remarks will explain my reasons for declining to adopt, at least for 

 the present, M. d'Orbigny's Spiriferina. I cannot but express my doubt of their being 

 any structural difference between it and Trigonotreta : they appear to be separated 

 merely by differences in the development of their respective structures, which it is 

 evident have little or no value in a generic point of view. 



Trigonotreta cristata, SchlotTieim. Plate VIII, figs. 9, 10, 11, 12, 13, 14. 



(?) OsTKACiTES CRISTA PTJLLi, Schl. Leonhard's Taschenbuch, vol, vii, p. 76, 1813. 



Teeebeatdlites cristatus, ScTil. Akad. Miinch., vol. vi, p. 28, pi. i, figs. 3 a, h, c, 1816. 



— — „ Petrefactenkunde, p. 265, 1820, 



— — ,, Boue, Edin. Phil. Journ., vol. xii, p. 144, 1825. 

 (?) — PECTEN ,, Boue, Loc. cit. 



Tekebeatula cristata „ De la Beche, Geol. Man., 1st ed., p. 384^ 1831 ; 



Germ. Transl. p. 459, 1832 ; and 3d Eng. ed,, 



p. 573, 1833. 

 Spieifer octoplicattjs, J. de C. Sow. Von Buch, Ueber Delthyris, p. 39, 1834. 

 Tekebeatdla ceistata, Schl. Phillips, Encyc, Met., vol, iv, p, 617, 1834, 

 Spieipee cristatus „ Quenstedt, Wiegmann's Arehiv, p, 79, 1835. 



Teeebratula ceistata „ Thomson, Outlines of Min, Geol,,&c,, vol, ii,p, 324, 1836, 

 Spieipee cristatus {partim), Schl. Le Cocq,, Mem, Geol, Soc, de France, t, iv, 



p. 185 (excluding the figures).'* 



^ The median plate is displayed in an unpublished figure of the internal structure of this species by Mr. 

 T, Davidson, 



2 Vide M'Coy's Synopsis of the Characters of the Carboniferous Fossils of Ireland, p, 127, fig. 14 c. 



' I have seen species with the median plate of a more rudimentary form than in Delthyris cyrtana ; 

 but I have not been able to ascertain their specific name. 



* The figures appear to be copies of J. Sowerby's Spirifer octoplicatus (Miu. Conch., pi. 562, figs. 2, 3). 



