BIVALVIA. 57 



Goldfuss, and the small example figured by him, tab. 136, fig. 5«, appears to be another 

 species altogether unlike the young examples of our shell. Trigonia Utterata 

 of Phillips, ' Geol. Yorksh.,' i, tab. xiv, fig. 11, from the Lias of Robin Hood's Bay, 

 is likewise a distinct species, and having the priority, that specific name must be 

 retained for it. Trigonia Goldfussii, and more especially young specimens, occur not 

 unfrequently in the coarse bed of planking forming part of the shelly beds of the 

 formation. Trigonia undulata from Fromberg, of which M. Agassiz has given two 

 very different figures, would appear to be nearly allied to our species, more especially 

 the shell figured by him, (Etudes Mol. Trigonees, tab. vi, fig. 1,) which exhibits small 

 tubercles upon the carina?. 



Localitg. Minchinhampton Common. 



Trigonia Moretoni. Tab. V, fig. 19, 19r/. 



? Trigonia conjungens, Phillips. Geol. Yorkshire, i, p. 122. 



Testa ovato-trigond, plano-convexd, umbonibus obtusis recurvis, area angustd, trans- 

 versim plicatd ; plicis magnis irregidaribus ; carina marginal i et interna depressis irregu- 

 lariter subnodidosis ; costis per series niimerosis {cetate jwniori arcuatis, adulto subangulatis,) 

 et tuberculatis, tuberculis posticis magnis crebris confuse dispositis. 



Shell ovately trigonal, rather depressed, umbones obtuse, recurved, anterior border 

 rounded, posterior border lengthened and slightly excavated; area narrow, transversely 

 plicated, plications large and irregular; marginal and inner carina? depressed, rather 

 obscure, (more especially in adult specimens,) irregularly undulated ; costae disposed in a 

 numerous series (about 16), which in the young state are regularly curved, but subse- 

 quently become somewhat angulated ; they are tuberculated, the posterior tubercles being 

 the larger, closely arranged and much confused or irregular. 



In the earliest stage of growth the aspect is so dissimilar that it requires a separate 

 notice, the shell is rather compressed, the costae are prominent, regular, and smooth, the 

 plications upon the area appearing like continuations of the costae, which they nearly 

 equal in size, and the oblique divisional line upon the area which replaces the median 

 carina is perceptible. 



It is only when five or six costae have been perfected, that they become indented, 

 the indentations becoming more strongly marked with succeeding costae, and at length 

 forming distinctly rounded tubercles ; during a series of five or wpwards, the tuberculated 

 costae continue to have a regular graceful curvature, but subsequently they become irregular 

 and confluent ; posteriorly the tubercles are large, and the costae are at that part usually 

 bent upwards at a considerable angle. Thus in the adult stage of growth, the surface 

 is always irregular and varies in every individual, even more than is usual in the tuber- 

 culated Trigonia'. It would seem to be more nearly allied to T. impressa than to any 

 other British species, but it is twice or thrice as large, has greater convexity, the apex is 

 more obtuse, the area has much larger and more distantly arranged plications, neither has 



8 



