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BRITISH FOSSIL TRIGONLE. 



T. denticulata has not resulted from any insufficient examination or lack of materials, 

 unless indeed it may be of specimens which are required to be exceptionally well pre- 

 served to enable us to estimate fairly the amount of variability which they possess. It 

 would have been easy to have increased the number of figures of such specimens upon 

 our plates ; the practical utility, however, of this would have been doubtful, and I content 

 myself with offering the present explanation, together with the following brief statement 

 of geological positions which have come under my observation. 



A fine specimen was obtained by Mr. Witchell in the highest bed of Supra-liassic 

 Sandstone, at Haresfield Hill, near Gloucester. I obtained the species in the same 

 position, and accompanied by T. formosa, in the celebrated Ammonite-bed at Frocester 

 Hill. In the Inferior Oolite of the Cotteswold Hills it is comparatively rare ; the only 

 specimens known to me were from the hard limestone of the Upper Trigonia-grit at 

 Rodborough Hill. In the same formation through the midland and northern counties it 

 appears to be a more common species. Upon the coast of Yorkshire at Blue Wyke the 

 Dogger has numerous ill-preserved costated forms, and also the Millepore-bed upon the 

 same coast, in a higher position, which should probably be referred to it, but hitherto 

 only doubtfully. The grey limestone and shale near to Cloughton, higher in position, 

 has produced numerous examples of different stages of growth, delicately preserved in a 

 thinly laminated soft shaly bed. Apparently also the species may be tabulated with the 

 Kelloway Rock, to the southward of Scarborough at Cayton Bay, but valves are rare and 

 ill preserved ; a specimen in my cabinet with the valves in position and free from com- 

 pression offers no distinction when compared with Inferior Oolite specimens. The small, 

 supposed variety from the Great Oolite of South Lincolnshire, having the general figure 

 somewhat shorter, and the habit gregarious, has been already noticed (p. 153). 



Trigonia elongata, Sow. Plate XXX. The typical form, figs. 3, 3 a, 3 b, 6. 



— — id., var. angustata, Lyc. Plate XXX, figs. 1, la, 2. 



— — id., var. lata, Lyc. Plate XXX, figs. 4, 5. 



Trigonia elongata of various authors ; for figures refer to the following works : 



Trigonia elongata, Sowerby. Min. Conch., tab. ccccxxxi, figs. 1, 2 (exclude fig. 3, 

 a distinct variety from France), 1825. 



— costata, var. Sowerby in memoir by Grant on the Geology of Cutcb, 



Trans. Geol. Soc, 2nd series, vol. v, pi. xxi, fig. 16, 1837. 



— elongata, Damon. Geo. of Weymouth, Sup., pi. ii, figs. 1, 2, 1860. 



— — Lycett. Suppl. Monograph Great Oolite Mollusca, Pal. Soc. for 



1861, p. 46, tab. xxxix, fig. 6, 1863. 



