206 BRITISH POSSIL TRIGONI^E. 



posteal portions ; these differences apply equally to the figure of T. concentrica given by De 

 Loriol and Pellat, ' Portl. de Boulogne/ pi. 8, fig. 2. The convexity of the last-named 

 figure is also much more considerable than our British Calcareous Grit species. 



Stratic/raphical Position and Locality. — The passage-beds over the Lower Calcareous 

 grit of Snainton, Yorkshire : accompanied by numerous Conchifera, including Trigonia 

 Snaintonensis and T. davellata. It appears to be rare, and only single valves have been 

 collected. Obtained by W. H. Huddleston, Esq. The name is from the Rev. J. P. Blake, 

 whose important contributions to the Geology and Palaeontology of the Northern Counties 

 of England are so well known. 



Trigonia concentrica, Ag. 



Since the notice of this species at p. 52 was written, no additional materials to 

 illustrate it have come under my observation. The specimens first examined and pro- 

 visionally assigned to it in 1870 were very imperfect, fragmentary, and altogether 

 insufficient to characterise the species. The hope that more satisfactory and less doubtful 

 specimens would be obtained not having been realised, it becomes necessary to remove 

 Trigonia concentrica from the ascertained list of British Trigoniae. 



A nearly allied species, obtained by Mr. Huddleston in the passage-beds over the Lower 

 Calcareous Grit of Snainton, may possibly be identical as a species with the fragmentary 

 specimens alluded to, A good example figured upon PL XLI, fig 4, under the name of T. 

 Blakei, has enabled me to correct the error upon page 52, and to describe a species of 

 the Lower Calcareous Grit distinct from the T. concentrica of Agassiz and of De Loriol. 



Trigonia paucicosta. Plate XI, figs. 8, 9 ; Plate XVI, fig. 7 ; Plate XXXVII, fig. 3, 



p. 57. 



Trigonia angulata. PL xiv, figs. 5, 6 ; pi. xxxvii, figs. 7, 8, 9, p. 54. 



Supplementary to the comparisons between these species at page 58, the number of 

 specimens of T. paucicosta since obtained have been so considerable as to illustrate the 

 distinctive difierences with much certainty. With a species so variable in its -surface- 

 ornaments numerous examples are necessary to exemphfy its aspects. I retain sixteen 

 specimens, and have examined probably not less than a hundred ; these, however, form but 

 an inconsiderable portion of specimens destroyed in endeavouring to separate them from 

 the hard Kelloway Rock at Cayton Bay. These numbers, collected over a very small 

 surface area, evince the gregarious habits of the species. Unlike the Scarborough shell, 

 the Inferior Oolite T. angulata was not gregarious ; it has occurred at various localities 



