UNDULATE. 67 



they are plain, but more frequently they are subtuberculated ; all of them commence at 

 the anterior border and cm've obliquely downwards ; the first few rows are simply curved 

 upwards at their posteal extremities to the carina ; those which succeed are more straight 

 and oblique ; their posteal portions form with the anteal portions a more decided angle, 

 which increases with every succeeding costa, until they form acute angles upon the middle 

 of the valve, the posteal portions passing upwards perpendicularly to the carina, but 

 without any increase in their size. In adult forms the last three or four posteal costae 

 pass downwards perpendicularly to the pallial border. 



Three Jurassic Trigoniaj are allied to T. V-costata. T. tripartita, Porbes, a much 

 smaller species, differs in having a few large posteal, straight, oblique costae, which are 

 distinct from the far more numerous and smaller anteal costae. The figure of T. anfjulata. 

 Sow., is much more produced, and attenuated posteally ; the hinge-border is more 

 lengthened and concave ; the umbones are more prominent and recurved ; the costae are 

 very much fewer, and posteally they form an undulation rather than an angle ; it is very 

 correctly represented by the coarse figure in the ' Mineral Conchology.' T. producta, 

 Lycett, is somewhat allied to it in the characters of the costae, but has the anteal series 

 few and distinctly tuberculated ; the general figure also is essentially different ; the anteal 

 not recurved apices, the lengthened and flattened area, together with the greater 

 general length of the shell, separate them very clearly. 



Adult specimens of T. V-costata have the length one sixth greater than the height ; of 

 small specimens the number is far more considerable ; these latter have been obtained at 

 numerous localities in beds of very different mineral character ; usually they have a more 

 lengthened figure transversely ; they differ materially one from another in the closeness 

 or wider separation of the costae. 



StratigrapJiical position and Localities. The figure given in the ' Annals of 

 Natural History ' was from a large example obtained in the Upper Trigonia-grit of 

 Rodborough Hill, near Stroud ; a few specimens have also occurred in a similar position 

 at several localities near to Cheltenham ; it is, however, rare throughout the Cotteswold 

 Hills. In Northamptonshire it occurs more commonly in the ferruginous beds, and is of 

 smaller dimensions. Numerous specimens (apparently dwarfed) also occur in the very 

 fossiliferous bed of the Dogger at Blue Wyke, near Robin Hood's Bay, Yorkshire ; they 

 are of various stages of growth ; three are depicted upon Plate XV. A considerable 

 number of small, imperfectly preserved Trigoniae also occur in the layers of oolitic slate 

 at Collyweston ; they are deprived of the test, and have probably undergone vertical 

 compression ; their condition, therefore, does not admit of a rigid comparison ; they also 

 differ much one with another in the general figure, and in the prominence or indistinctive- 

 ness of the costae ; all have the appearance of young shells, and occasionally specimens 

 have the figure more lengthened transversely than is observed even in young examples of 

 T. V-costata ; there does not, hovi'ever, occur any constant characters which will justify 

 their separation from that species, to which, therefore, they are provisionally united ; two 



