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BRITISH FOSSIL TRIGONLE. 



certain of the Upper-Jurassic Glabra they have a space, anterior to the divisional angle 

 of the valve, devoid of the costae, which occupy the anteal portion of the shell, but 

 possessing a feature peculiar to this group, inasmuch as the space is occupied by a 

 series of faintly defined perpendicular costellae which are directed upwards, some from 

 the pallial border, others from the posteal extremities of the costae, to the angle of 

 the valve ; the area and escutcheon have also their transverse costellae ; the whole of 

 this ornamentation is slightly crenulated ; it may be designated the pennata group. 



Trigonia pennata, Sow., shell ovately trigonal, convex; umbones antero-mesial ; 

 only slightly recurved, not prominent ; anterior border rounded, produced ; lower 

 border lengthened, curved, its posteal extremity pointed ; siphonal border sloping 

 obliquely ; escutcheon rather wide, depressed, its border nearly straight. Area wide, 

 flattened, its surface forming a considerable angle with the other portion of the valve ; 

 its surface is traversed transversely by small rows of scabrous costellae, which curve 

 obliquely from the angle of the valve, pass uninterruptedly across a slight elevation 

 which forms the upper boundary of the area and are continued across the escutcheon. 

 The other portion of the valve has the costae (about thirty in fully developed forms) 

 prominent, regular, nearly horizontal, and closely arranged ; posteally their extremities 

 form nearly right angles with the lower extremities of a much smaller, more faintly 

 defined, perpendicular series of costellae, which terminate upwards at the divisional 

 angle of the valve throughout its entire length ; both costae and costellae are minutely 

 crenulated. The posteal costellug and those of the area diverge from the angle of the 

 valve, as stated by Mr. Sowerby, like the rays of a feather. 



D'Orbigny ('Pal. Fran., Terr. Cret.,' vol. iii, p. 150) made T. pennata a synonym of 

 T. sulcataria, Lam., an error which probably originated in the very incorrect drawing 

 of Sowerby's species in the f Mineral Conchology/ where the apex of the valve is 

 represented as considerably produced and recurved ; there is also apparently a divisional 

 line or ridge crossing the valve obliquely and intersecting the rows of costae at the 

 point where they are united to the extremities of the smaller or perpendicular costellae. 

 Neither of these characters exist ; in fact, the artist appears in some measure to have made 

 up the great imperfections of the type-specimen with an ideal representation. The type- 

 specimen now in the British Museum is very defective, and contrasts strongly with the 

 original drawing ; its size agrees nearly with the smaller of our figures. Compared 

 with T. sulcataria, T. pe?inala is a much smaller species, differing from the former both 

 in the figure and ornamentation ; both have the series of scabrous costae anteally, but 

 T. pennata has the general figure less inflated and more lengthened ; the umbones are 

 less elevated and less recurved, the area supplies the most striking distinctive feature ; 

 it is regularly transversely costellated almost its entire length. In Lamarck's species it 

 is smooth, excepting near to the umbones. The latter species has the costae shorter, 

 more oblique, and more distantly arranged j the series of small perpendicular or 



