﻿UNDULATiE. 81 



gular aspect exhibited in all the specimens figured by Agassiz. The figures of 

 Quenstedt have the ornamentation approximating more nearly to our species, but the 

 general figure coincides with the shell of Agassiz, and cannot be identified with T. 

 Sharjnana. Various specimens from Normandy in the British Museum from the Inferior 

 Oohte at St. Vigor and Montiers have an absolute specific identity with the British 

 •examples, and serve materially to establish their distinctness from T. ^iulcUella. 



Trigonia costatula, Lijc. PL XV, figs. 8, 9, 10 ; PI. XII, fig. G, 6a. 



Trigonia costatula, Lycett. Ann. and Mag. Nat. Hist., 1850, p. 421, tab. .\i, fig. 5. 



— EXiGUA, Lycett. Ann. and Mag. Nat. Hist., 1850, tab. xi, fig. 3 (young 



e.xample dwarfed). 



— COSTATULA and T. exigua, Morris. Catal., 1854, p. 228. 



Shell convex, ovately trigonal, or sub-quadrate ; umbones elevated, obtuse, sub- 

 mesial, scarcely recurved ; anterior side produced, its border, together with the lower 

 border, curved elliptically. Escutcheon narrow, short, and depressed ; only slightly more 

 lengthened than the posterior border of the area, with which it forms an obtuse angle. 

 Area wide, flattened, divided by an oblique mesial furrow, and bounded by two incon- 

 spicuous, knotted, small carinse ; the knots upon the inner carina assume the form of 

 small varices, which are occasionally somewhat extended upon the escutcheon ; the inter- 

 carinal space is occupied by small, irregular, transverse plications, which are sometimes 

 only faintly traced. The other portion of the shell has a numerous series (21 — 22) of 

 smooth, narrow, horizontal, or somewhat concentrically curved costa?, which near to the 

 lower border become less elevated, and have also less regularity ; a large specimen of the 

 left valve has the last-formed two or three costse more or less broken posteally ; other 

 specimens have their last-formed four or five costse attenuated posteally, and bent slightly 

 upwards with some irregularity to the marginal carina. 



Trit/onia exigua was founded upon very perfect examples of the young shell of the 

 present species, obtained with numerous other dwarfed and immature testacea in the 

 shelly freestone of Leckhampton Hill, near Cheltenham. In this young condition the first- 

 formed costse, to the number of thirteen or fourteen, are united almost uninterruptedly to 

 the knotted elevations which constitute the marginal carina ; they then pass across 

 the area in the form of smaller costellse, each of which terminates at the slight nodosities 

 which form the inner carina; occasionally an intercalated costella is formed upon the 

 area, which also possesses only slight indications of a median furrow; the uniformity, 

 close arrangement, elevation, and acute edges of these first-formed costse are remarkable, 

 and differ greatly from the condition of specimens of more -developed growth, obtained in 

 a bed of hard cream-colonred limestone (coralline mud), in which specimens have been 

 cleared with great difficulty with the help of cutting instruments. Due allowance being 



