14 BRITISH EOSSIL TRIGONE. 



§ I. ScAPHOIDEjE. 



Trigonia duplicata, Sow. Plate I, figs. 8, 9, 10. 



Trigonia duplicata, Sow. Min. Con., pi. 237, fig. 4. 



— Proserpina, D'Orb. Prodr., 1, 10 Et., No. 315, p. 278. 



— duplicata. lb., No. 317, p. 279. 



— — Morris. Catal., 1854, p. 228. 



— — Morr. and Lye. Pal. Soc. Mon., 1854, pi. vi, fig. 2. 



Shell ovately trigonal, moderately convex ; umbones antero-mesial, elevated, obtuse, 

 and somewhat recurved ; anterior side much produced, its border curved elliptically with 

 the lower border, posterior extremity more produced, attenuated, and rostrated ; hinge- 

 border concave, sloping obliquely downwards ; area narrow, flattened, finely striated 

 transversely, and bounded by two very small carina3, each of which is minutely tuber- 

 culated ; escutcheon narrow, depressed, and lengthened, forming an excavation at the 

 superior border ; it has delicate oblique costellae, and at the apical extremity are a few 

 transverse ridges, which also pass over the inner carinas and the area. Costae numerous 

 (from 12 to 14), narrow, raised, delicately fringed with closely [placed, minute, obtuse, 

 rounded, or ovate tubercles ; the first-formed three costae are concentric, the fourth costa 

 is directed downwards, and has anteally to it four or five short irregular horizontal or 

 supplementary costae, which are nearly at right angles with the fourth costa ; the suc- 

 ceeding costae — nine, ten, or more in number — pass from the carina downwards in a 

 straight or slightly waved course perpendicularly to the pallial border ; two, three, or four 

 of the more anteal of these costae divide near to the border or near to the middle of their 

 course, each into several smaller costae, but the few more posteal costae are undivided. 

 These small supplementary pallial costae are never precisely alike in any two specimens, 

 but are always small and numerous. The inner borders of the valves are crenulated. 



Few Trigoniae have so much variability as T. duplicata, both in the general figure and the 

 arrangement of the rows of costae ; the number of the rows, their closeness or separation, 

 and more especially the number of the small supplementary costae, all are characterized by 

 this diversity ; usually the smaller specimens have the greater convexity, and have their 

 apices more produced and recurved. D'Orbigny was misled by these differences, as 

 exemplified by certain specimens, to separate his T. Proserpina ; and for some time the 

 larger and more depressed forms, with their closely placed costae, induced me also to 

 regard them as distinct ; the figures now given will sufficiently illustrate this variability. 



Agassiz placed T. duplicata with the Scabra, influenced, probably, by the examination 

 of insufficient specimens ; it is only necessary to direct attention to the characters of the 



