﻿52 BRITISH FOSSIL TRIGONIiE. 



Trigonia concentrica, A(/. (See figures upon the kst plate of this Monograph.) 



Trigonia concentrica, Agassiz. Mem. sur les Trigonies, 1840, p. 20, tab. 6, fig. 10. 

 Ltriodon concentricuji, Bronn. Index, Paleont., 1848, p. 685. 

 Trigonia concentrica, D'Orbigny. Prodrome, 1350, vol. ii, p. 50, No. 121. 



— — Pictet. Traite de Pal6ontologie, 1855, p. 539. 



— SUBCONCENTEICA, jE'^fl?/o>i. Thurmanii, 18G2, LetheaBruntrutana, p. 203, pi. 25, 



fig. 6. 



— CONCENTRICA, de Loriol et Pellat. Paleont. et Geol. de Etage Portlandien, 



1866, pi. 8, fig. 2. 



Shell ovately trigonal, moderately convex ; umbones small, antero-mesial, not recurved, 

 and but little produced, borders of the valves rounded ; area narrow, flattened, distinctly 

 bipartite, with irregular and unequal transverse plications, and bounded by two rows of 

 regular, rounded, small tubercles ; the escutcheon is of moderate size, excavated, its 

 superior border is raised. The other portion of the valve is characterised by numerous 

 rows (thirteen in small specimens) of regular concentric costie, with small, closely arranged, 

 rounded or ovate tubercles. These become somewhat cord-like, and attenuated anteally ; 

 both extremities of the costse curve gently upwards ; their posteal tubercles are conformable 

 in size with those upon the marginal carina, Avhich they meet at right angles. 



Our specimens, which are dwarfed and not of adult growth, were procured in the 

 Coralline Oolite of Steeple Ashton, Wilts ; they agree in their general aspect more nearly 

 with the figure of de Loriol than with that of Agassiz. The minutely tubei'culated, 

 closely arranged, concentric costse distinguish them readily from T. coraUina. 



Length, 10 lines; opposite diameter, 6^ lines ; diameter through the united valves, 

 5 lines. 



The foreign localities cited are Laufon (Bale), Switzerland ; St. Jean-d'Angely, 

 Auxerre (Yonne), Sennantes (Oise), France ; all in the lower Portlandian or Kimmeridge 

 strata. 



