58 MOLLUSCA FROM THE GREAT OOLITE. 



it the distinct and regularly tuberculated marginal carina of that species ; the arrangement 

 of the rows of costae is similar, but the tubercles are larger, and the adult condition more 

 confusedly disposed in our shell. 



It occurs in the shelly beds of the Great Oolite, in which small specimens are 

 abundant, but adult forms are comparatively rare. The species is respectfully dedicated 

 to Lord Moreton, who has assiduously cultivated geological science. 



Localities. The Minchinhampton district in general. Stonesfield slate, Oxfordshire. 



Trigonia costata, Sow., Far. pullus. Tab. V, fig. 22, 22a. 



Curvirostra non rugosa, Luid. Lithoph. Brit., t. 9, No. 714, 17(50. 

 Donacites costatus, Schoth. Petref., i, p. 193, 1820. 

 Lyriodon costatum, Bronn. Leth. Geog., t. 20, f. 4, 1836, 1851. 

 Lyrodon costatum, Goldf. Petref., ii, t. 137, f. 3, 1840. 

 Trigonia pullus, Sow. Min. Conch., t. 508, 1826. 



— costata, Lamarck. An. s. Vert., vi, p. 64, 1819. 



— — Parkinson. Org. Rem., iii, t. 12, f. 4, 1811. 

 _ _ Zeitea. Wurtt., p. 78, t. 58, f. 5, 1834. 



_ _ Roemer. Oolith., p. 97, 1835. 



— Agassiz. Mem. sur les Trigonees, p. 35, t. 3, f. 12 — 14, 1S40. 



Testa subtrigond, umbonibus prominentibus recurvis, acutis, area magna, plicata et 

 carinatd. 



In delate juniori, carina marginali acuta et laevigata, carina media et interna denticulatd. 

 JState adulto area in valvd sinistra carina marginali magna rotundatd et indentatd ; 

 carina media et interna distinctd et denticulatd sed parvd ; superficie inter carinis plicis 

 longitudinalibtes densis interdum spinis acutis instructis ; area in valvd altera sine carina 

 media plicisque lungitudinalibus magnis, paucis intertiisque latis et profundus. Costis 

 dorsalibus magnis, lavigatis, eleoatis et curvatis, carina marginali separatis. 



Shell subtrigonal, umbones prominent, acute, recurved; area large, longitudinally 

 plicated and carinated, dorsal surface costated. In the young state, the marginal carina 

 is acute and smooth, the inner and mesial carinas are denticulated. In a more advanced 

 stage of growth, the area in the left valve has a marginal carina large, rounded, and deeply 

 indented, the median and inner carina; are distinct and denticulated, the spaces between 

 the carinse have numerous longitudinal plications, which are not unfrequently covered 

 with asperities, or acute spinous elevations. In the right valve, the surface of the area is 

 different ; it is divided into two portions, the posterior portion being more depressed than 

 the other, there is no distinct median carina, but the anterior portion of the area has two 

 large indented plications, separated by wide interstitial spaces, ultimately two other 

 plications are added. The longitudinal costse are large, smooth, and gracefully curved, 

 separated from the marginal carina by a smooth sulcus ; the lanceolate space between the 

 inner carina; has a surface very similar to that of the area. Notwithstanding the frequency 



