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BRITISH FOSSIL TRIGONLE. 



Another allied form which has sometimes been mistaken for T. spinosa is T. ornata, 

 D'Orb. ; the latter species, which is limited to the Neocomian formation, will also be 

 found figured and described in the present Monograph. Compared with T. spinosa, and 

 represented by an uncompressed specimen, it will be seen to have much greater con- 

 vexity ; the umbones are larger and much more recurved ; the divisional angle of the 

 valve has greater prominence ; the surface of the area is steeper and narrower ; the 

 escutcheon is wider and more excavated ; the pallial costae have much greater curvature ; 

 they are without obtuse spines, having their rounded surfaces only crenulated ; their 

 attenuated extremities also form a sinuation as they pass upwards to the angle of the 

 valve ; the perpendicular plications occupying the intercostal spaces are also unusually 

 prominent. 



A little Trigonia figured by Nilsson under the name of T. pumila (' Petref. Sulc.,' 

 tab. 5, fig. 7) has been referred to by Pusch as probably representing a young specimen 

 of T. spinosa ; the figure does not appear to be a satisfactory representation of Parkinson's 

 species ; the small portion of the area visible indicates that its surface forms a consi- 

 derable angle with the other portion of the valve ; the costae differ in being without 

 distinct spines, and in having a considerable curvature. The general aspect agrees better 

 with a small example of T. Archiaciana. 



T. Lamarckii, Matheron (' Catal. des Corps org. foss. du Depart, des Bouches du 

 Rhone,' pi. 22, figs. 5 — 7), possesses some general resemblance to T. spinosa in the 

 outline and in the arrangement of the spinose costae ; but the convexity is more con- 

 siderable ; the area is more excavated ; the escutcheon has greater breadth ; the costae 

 have greater curvature and much less prominence, their spines are much smaller, more 

 numerous, and more pointed. 



The British Museum possesses the unusually fine specimen of T. spinosa figured in the 

 ' Mineral Conchology,' numbered " 50,003 ;" but it is equalled in size by other examples in 

 the same collection. Our figures represent much smaller but equally characteristic shells ; 

 the two specimens on Plate XXIV are from the Greensand of the Blackdown region ; 

 apparently it is absent in the more western outliers of the same formation at Great and 

 Little Haldon. 



The specimen figured (Plate XXIII, fig. 10) is from the Upper Greensand of the 

 Isle of Wight ; it is somewhat more lengthened and has greater convexity, but differs in 

 no other feature ; it is the sole example having the test preserved and obtained in the 

 Upper Greensand which has come under my notice ; specimens from that formation 

 without the test, of various dimensions, are not uncommon, both in the Isle of Wight and 

 near to Devizes ; from the latter place Mr. Cunnington kindly forwarded numerous 

 specimens. The usual condition of these external moulds is very defective. For the 

 most part they are flattened from vertical pressure ; their costae have little prominence, 

 and their spines are indicated only by slight irregularities upon their upper borders. It 

 is intended to figure two of the larger of the moulds on Plate XXVI 1 1. 



