80 BRITISH FOSSIL TRIGONIiE. 



valve, wliere they are suddenly bent horizontally forwards, and form a slight undulation 

 nearly to the anterior border; the last-formed three or four rows descend perpendicularly 

 to the lower border; all the rows have great regularity in their arrangement, and are 

 nearly of equal size throughout their course. 

 The internal mould is unknown. 



Stratigraphical position. — A specimen unusually large but without name as a 

 species was figured by Morton in his ' Natural History ' of Northamptonshire. 

 The figure gives the characteristic features with truthfulness and minuteness, and 

 is the only notice of it which T have discovered. Our smaller figures represent 

 the usual size of specimens preserved in the form of external casts upon slabs of 

 the sandy iron oolite of Northamptonshire. These preserve the more minute features 

 of the Trigonia with great delicacy ; for examples I am indebted to the kindness 

 of Mr. S. Sharp, of Dallington Hall, Northampton, who has investigated the 

 geology and palaeontology of his district with long-continued perseverance and success. 

 His collection has the only few Northamptonshire specimens with the tests preserved 

 which I am acquainted with ; my own cabinet has a single much larger example, but of 

 less mature growth, from the shelly bed of the Dogger at Blue Wyke, near Robin Hood 

 Bay, Yorkshire. The coarse ferruginous Oolite of Glaizedale in the same county also 

 very commonly contains external casts of this Trigonia under circumstances of lithological 

 character closely resembling the beds in Northamptonshire, and in like manner associated 

 with Astarte elegans. Sow., A. rhomboidalis, Phil, and other well-known testacea of the 

 Inferior Oolite. The Glaizedale casts of the Trigonia are much larger than those of 

 Northamptonshire. It, therefore, appears to be a characteristic shell of the lower portion 

 of the Inferior Oohte in the midland and northern counties of England. 



Affinities. It is allied to T. compta, from which it is distinguished by having the area 

 more expanded, by the absence of the few large isolated varices at the posteal extremities 

 of the costse, by their greater upward posteal curvature, and by their an teal undulation. 

 Trom T. Phillipsi it is separated by the great breadth of the area with its distantly 

 arranged transverse costellse, together with the undulation and occasional angularity in 

 the curvature of the rows of costae. 



1\ Sharpia7ia has also considerable afiinities with a still smaller species, viz. the 2\ 

 pulchella, of Agassiz (' Trig.,' pi. xxv, t. 2, figs. 1 — 7), from strata, which he assigns to 

 Upper Lias, at Urweilar and Miihlhausen (Haut-Rhin). Subsequently Quenstedt 

 figured this little species in his ' Handbuche der Petrefacten ' (tab. 43, fig. 14), and assigned 

 it to the lowest zone of the Inferior Oolite. Compared with the British species all these 

 forms are more oblong or sub-quadrate ; their umbones are therefore more anterior, and 

 are scarcely raised above the superior border. In T. Sharpiana the umbones are much 

 raised, and have so much prominence that the general figure is ovately trigonal ; the 

 number of tuberculated costse are variable, and diff'er much in their mesial angularity or 

 undulation, but they are always more numerous, and never assume the broken and irre- 



