94 BRITISH OOLITIC AND LIASIC BRACHIOPODA. 



do not generally attain to the frontal margin, but disappear at a little distance from it, 

 so that the lateral plaits of the mesial fold and the side plaits are often widely separated 

 by a large flat space ; sometimes the lateral costse are observed to bifurcate. 



Dimensions variable; the largest specimen I have seen, measured, length 15, width 21, 

 depth 15 lines ; but in general they do not exceed length 12, width 13, depth 13 lines. 



Obs. R. tetraedra is one of those perplexing species often difficult to determine, from 

 the great variety it presents, owing to several causes, and especially to the variable number 

 of plaits ornamenting its surface, and mesial fold, which sometimes is considerably pro- 

 duced above the sides of the valve, while in other cases very little difference of level is 

 perceptible, the mesial fold, as well as sinus, being shallow. Mr. J. de C. Sowerby having 

 kindly placed at my disposal for examination the original types of T. tetraedra and media, 

 with the assistance of Mr. Waterhouse, we came to the same conclusion, already arrived 

 at by some authors (V. Buch, 1 Morris, Bronn, &c), viz., that T. media of Sowerby is only 

 a variety of tetraedra. In true media, as seen by the profile view, PI. XVIII, fig. 9, 

 the larger valve is deeper in the middle, and the beak more recurved, touching the umbo ; 

 the shell being, in general, more globose than what we observe in the specimens of true 

 R. tetraedra, Sowerby, Plate XVIII, fig. 8 ; but, on the examination of a suite of specimens, 

 we soon perceive that there is every degree of passage between the extreme states of the 

 same species. I cannot, however, admit, with Professor Bronn, 2 that T. obsoleta, of 

 Sowerby, is a synonym of tetraedra ; both forms seem to me sufficiently distinct to be 

 retained as separate species. R. obsoleta is a much less convex, and rather compressed 

 shell, the longitudinal curve of the valves being more uniformly regular ; the beak not so 

 recurved, always exhibiting under it the foramen, entirely surrounded by the deltidium, 

 which is separated more or less from the umbo. The same author's other synonyms of the 

 present species would likewise require further examination before being admitted, as there 

 exists some doubts as to the true types of some of Zieten's forms, as well as Mr. Quenstedt's 

 rectifications ; 3 indeed, I am sorry to find, that some German authors have added much 

 to the confusion by attributing to the Sowerby types species widely separated both in 

 form and stratigraphical position. Lamarck, as justly remarked by Baron V. Buch, 

 confounded the species under consideration with Schlotheim's R. decorata. 4, Kcenig 

 committed the same error. 5 R. decorata has not, to my knowledge, been found in the Lias 

 or Cretaceous formations, as stated by the last-named author. W. Smith's figure of 

 T. media 5 seems more likely to be R. varians of Schlotheim. 



1 See for reference at the head of this description. 



2 Index Pal., p. 1258. 



3 Das Flbzgeberges Wurtembergs, 1843. 



* See Lamarck, 'An. Sans. Vert.,' vol. vi, 1819, and Von Buch, 'Mem. Soc. Geol. de France,' 

 vol. hi, p. 146. 



5 Icones Fossilium Sectiles, p. 3, pi. vi, fig. 72. 



6 Strata identified by organised Fossils ; London, June, 1816 — 27, pi. xix, figs. 1 — 3. 



