58 MOLLUSCA FROM THE GREAT OOLITE. 



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

 of the rows of costse 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, 22«. 



CuRViROSTRA NON RUGOSA, Luid. Lithoph. Brit., t. 9, No. 714, 1760. 

 DoNACiTES cosTATUS, Schoth. Pctrcf., 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. 



— — Zeiten. Wurtt., p. 78, t. 58, f. 5, 1834. 



— — Roemer. Oolith., p. 97, 1835. 



— — Agassiz. Mem. sur les Trigone'es, p. 35, t. 3, f. 12 — 14, 1840. 



Testa suhtrigond, umhonihus prominentibus recurvis, acutis, area magna, plicata ef 

 carinatd. 



In atate junior i, carind marginali acutd et Icevigatd, carina rtiedid et interna denticulatd. 



jEtate adulto area in valvd sinistra carind marginali magnd rotundatd et indentatd ; 

 carind medid et internd distinctd et denticulatd sed parvd ; superjicie inter carinis plicis 

 longitudinalibus densis interdum spinis acutis instructis ; ared in valvd alterd sine carifid 

 medid plicisque longitudinalibus magnis, paucis intertiisque latis et profundis. Costis 

 dorsalibus magnis, lavigatis, elevatis et curvatis, carind 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 carinse 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 carina3 are distinct and denticulated, the spaces between 

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

 with asperities, or acute spinous elevations. In the right valve, the siu'face 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 costae are large, smooth, and gracefully curved, 

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

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



