ADDENDA. 



211 



Trigonia conocardiiformis, Krauss. Inner surface of valve. 



Comparison oj ike TrigonicB of the Blackdoicn Beds with those of Bracquegnies. — The 

 Whetstones (meule) of Bracquegnies, Belgium, are upon the same horizon, and are 

 identical lithologically, for the most part, with the Blackdown Whetstones (Cornet and 

 Briart, " Description de la Meule de Bracquegnies," ' Memoires Couronnes et Mem. des 

 Savants Strangers,' Acad. Royale de Belgique, t. xxxiv, 1868). Like to the British 

 deposits, they are characterised by the prevalence of Trigonia dadalea. Park., Cornet 

 and Briart, pi. 6, figs. 1, 3. The numerous specimens have individual peculiarities, but 

 they all differ from the usual Blackdown form, of which good illustrative examples are 

 given upon our Plate XXIII, figs. 2 and 3, which represent specimens of full dimen- 

 sions ; the Belgian specimens are identical with our large variety confusa (Plate XXIII, 

 fig. 1, p. 102). In Britain this variety is comparatively rare, and is found a little 

 higher in stratigraphical position. In Devonshire it has occurred only at Little Haldon, 

 at the base of the Upper Greensands, in a pebbly bed special to that region. The 

 British Museum has upon its tablets a fine and varied series of adult forms of this large 

 variety, which are exhibited as examples of T. dadalea. I have placed it as a variety, 

 but possibly other observers may be inclined to regard it as a distinct species. The 

 most prominent varietal features consist in the unusually large, confused, rounded 

 tubercles, which cover and crowd the larger or posteal portion of the shell, and in the 

 large tuberculated escutcheon, rendering the carinal nodes indistinct or only wxll defined 

 near to the umbones. 



A series of the Bracquegnies specimens, kindly forwarded to me by Dr. C. Barrois, 

 of Lille, illustrates every stage of growth in the variety confusa. The few first-formed 

 rows of costae are plain and angulated or nearly destitute of nodes or tubercles, 



