340 MR. S. S. BTTCKMAN ON BRACHIOPOD MORPHOLOGY. [Aug. I967, 



trigonella, or Trigonella suevica, 1 and by A. d'Orbigny Terebratella 

 Eleuriausa. 2 



It is fairly obvious that this shell is not in genetic connexion 

 with the Lias examples of the quadrifid stage ; and yet, though the 

 species shows no trace of any prior stage, it is reasonable to conclude 

 that the shell must have arrived at the complete quadricarinate 

 form by passing through quadrifid and Cincta-st&ges. 



There is a Muschelkalk shell which is a good example of the 

 quadricarinate stage, presenting, too, an interesting case of hetero- 

 chronous homceomorphy. It is Tetractinella trigonella (Schlotheim) 

 Bittner, which is externally so exactly similar to the White-Jura 

 trigonellid that for a long time the two were united under the 

 one name Terebratula trigonella, though they were known to come 

 from formations so wide apart as the Muschelkalk and the White 

 Jura. However, their internal arrangements show much difference 

 — the Muschelkalk shell is a spire-bearer, the White-Jura fossil a 

 long-looped Terebratuloid. 



There is yet a further stage of development after the quadri- 

 carinate : it may be called the multicarinate or pectunculus- 

 stage, after its example in the White Jura, Trigonellina pectunculus. 3 

 In this stage there is multicarination, obtained by the development 

 of intermediate carinas between the principal carinas of the trigonellid 

 stage. So Trigonellina pectunculus is really a trigonellid in youth, 

 and only attains the true pectunculus- stage in maturity. In its 

 nodose costas, it shows that it is making a beginning towards the 

 spinose stage. 



The names given to the various stages refer only to the form and 

 ornament of the valves : they have nothing to do with the loop. 

 It will be seen, however, that Cincta is a simple form so far as shell 

 goes, and that out of such a form there is a whole series of possible 

 developments. Yet Cincta, which is comparatively primitive in 

 shell-form, has a somewhat advanced loop : this is in the dallini- 

 form stage. On the other hand, Trigonellina, which is two stages 

 in advance of Cincta, so far as shell-form goes, is two stages behind 

 Cincta in its loop : that is in the muehlfeldtiform stage. The fact 

 that T. pectunculus has been referred to the genera Terebratella, 

 Megerlia = Muehlfeldtia, and Ismenia, shows that its loop is of an 

 earlier type than that of Cincta. 



It is interesting to see how very different is the development, 

 and the association of characters in the genus Eudesia. In Eudesia 

 there is a highly-developed multicarinate stage — the carinas being 

 so numerous as to make the term multicostae more appropriate ; 

 but all these carinas are alternate, not opposite. Eudesia therefore, 

 by its shell-form is higher than Cincta, in a way even higher than 



1 ' Petrefaktenkunde Deutschlands ' vol. ii (1868) ' Die Brachiopoden ' p. 25. 



2 ' Prodrome de Paleontologie ' vol. ii (J 850) p. 25. 

 2 See Appendix, p. 342. 



