90 BRITISH CRETACEOUS BRACHIOPODA. 



the ventral or dental one with a wide longitudinal sinus, to which corresponds a slightly 

 raised mesial fold in the opposite or dorsal valve : beak acute, tapering and but slightly 

 incurved, ridges sharply defined, leaving a wide flattened space or false area between them 

 and the hinge line, which last indents the lateral portions of the umbo : foramen compara- 

 tively large, entirely surrounded, and more or less removed from the hinge line by the 

 deltidium and its tubular prolongations • lateral margins moderately sinuous ; the frontal 

 edge of the ventral valve indents the opposite one to a lesser or greater extent. Externallv, 

 17 to 30 plaits ornament each valve, 6 to 10 forming the mesial fold. Dimensions 

 variable : length 8, width 9, depth 6 lines ; 

 7 7i 4 



Obs. The shell above described has been distinguished and admitted by British 

 geologists as Ter. depressa of Sowerby, 1 although misunderstood by several continental 

 authors. It may, however, remain a question whether R. depressa be really distinct 

 from the Anomites triangularis of Wahlenberg, 2 a point I have been unable to determine, 

 from the figures published by the Swedish author not conveying a sufficiently satisfactory 

 resemblance to Sowerby's species and specimens, being too circular, and exhibiting no 

 trace of mesial fold or sinus, which is always visible in examples of similar dimensions of 

 R. depressa. Nilsson 3 describes and reproduces Wahlenberg' s figures, but does not throw 

 further light on the contested question. The beak, foramen, and deltidium are both inac- 

 curately and vaguely represented, for which reasons I did not consider it advisable to remove 

 Sowerby's denomination until more positive evidence can be obtained by the inspection of 

 Swedish specimens. 



Mr. Sharpe considers that among the shells found at Farringdon, and referred by 

 myself to R. depressa, the two species do occur, and may be distinguished ; but after 

 a minute study of all the specimens collected by that distinguished Palaeontologist, as well 

 as of those assembled in the locality by myself and others, I felt unable to arrive at a similar 

 conclusion, from finding that all possessed (to my eyes) the same essential specific character. 

 According to Mr. Sharpe, the young shell (PI. XI, fig. 32) would represent R. triangu- 

 laris, while the figs. 29 and 30, represent R. depressa ; in these, however, we observe the 

 same general shape, the same character of plication, with many of the plicae augmenting by 

 intercalation, and varying in number; a peculiarity common to specimens of every species 



1 " Triangular, depressed, regularly plaited, front elevated, lateral angles rounded, beaks produced, 

 plaits 20 ; when so young that the front is hardly elevated, this shell is almost orbicular : in whicb circum- 

 stance it differs from the last {Ter. lata), the proportions of which do not vary much by age ; the plaits 

 are sharp, about eight of them are raised with the front. Found at Farringdon. ('Min. Con.,' vol. v, p. 165, 

 tab. i)ii, fig. 2.) 



2 ' Petrifacta Telluris Suecana, Nova Acta Regise Societates Scientiarum Upsaliensis, vol. viii, tab. iii, 

 figs. 11, 12, 13, 1821. 



3 'Petrifacta Suecana,' p. 36, tab. iv, fig. 10, 1827. " T. testa ovato-triangulari, longitudinaliter 

 sulcata; sulcis et interstriis numerosissimis sequalibus : valva minore convexiore ; rostro acutangula sub- 

 recto ; margine stiperiore ; hex sinuato. Locality — Balsberg," where it is stated to be rare. 



