CLAVELLAT.E. 35 



its area is much more nearly upon the same plane as the other portion of the surface ; its 

 marginal carina is, therefore, more remote from the superior border of the valve. The 

 rows of tuberculated costsc are much more numerous, and their tubercles are also smaller, 

 so that when viewed in certain directions the rows appear to take a different direction, 

 and to be nearly vertical. The tubercles in their figure accord with those of the Inferior 

 Oolite species ; their bases are compressed laterally, and touch the row of tubercles next 

 in succession ; but it is only in T. Griesbachi that their close proximity produces this 

 deceptive appearance of a vertical arrangement in the rows. 



The area is flattened, narrow, with two very small tuberculated, bounding carinas, 

 and with acute, transverse costellae, every alternate one of which forms a small varix upon 

 the inner carina, and is prolonged somewhat upon the escutcheon. 



Stratigraphical position and locality. The late Rev. A. W. Griesbach obtained this 

 remarkable little species in the Cornbrash of Rushden, Northamptonshire. It has also 

 occurred in the upper zone of the Great Oolite, near to Cirencester. 



Trigonia Formosa, Lye. Plate V, figs. 4, 5, 6. 



Trigonia striata, Quenstedt. Jura., 1857, tab. 46, fig. 2. 



— Formosa, Lycett. Jour. Geol. Soc, 1859, note in Memoir of Wright on the 



Inferior Oolite formation. 



Shell ovately trigonal, depressed ; umbones elevated, pointed, and recurved ; anterior 

 side moderately produced; both it and the lower border elliptically curved ; superior 

 border lengthened and concave ; area rather narrow, flattened, with closely arranged 

 acute, transverse striations ; a faintly marked oblique, mesial furrow, and bounded by 

 two small densely and minutely dentated carina) ; the escutcheon is concave, smooth, and 

 lengthened, sloping obliquely downwards, forming a considerable angle posteally with 

 the posterior extremity of the area. The costated portion of the shell has very numerous 

 narrow, oblique, knotted ridges, Avhich are small at the carina, but increase in size 

 anteally, where they also curve more or less horizontally, even to the anterior border ; the 

 last-formed five or six ridges arrive at the pallial border almost without curvature. 



The umbonal extremity of the area has costellae in lieu of transverse striations. This 

 well-characterised species was long confounded with T. striata, Miller, owing probably to 

 the bad figures originally given of the latter species ; as a contrast to these the Trigonia 

 Montierensis figured by Goldfuss under the name of T. striata is excellent. Upon comparing 

 examples of equal size it will at once be observed that the general figure is very different ; 

 T. striata is by do means depressed like the other; its superior border is short, straight, 

 and nearly horizontal, so that its posteal extremity is at less than half the distance from 



