BIVALVIA. 55 



flattened, or excavated area, which extends posteriorly from the umbo to the infero-posterior 

 extremity, and is separated from the dorsal surface by a ridge or angle, and by a similar 

 division from a small lanceolate space upon the other side, the anterior part of which 

 supports the ligament ; umbones recurved, contiguous, usually angulated ; the dorsal 

 surface is ornamented with longitudinal or concentric rows of costae or tubercles. The 

 hinge has four oblong compressed diverging teeth in one valve, the sides of which are 

 grooved transversely, and two similar teeth in the other valve; ligament external, 

 muscular impressions two in each valve, elliptical, and deeply impressed. 



In the descriptions of species, we use the term marginal carina to indicate the ridge 

 which bounds the area from the other surface of the shell; inner carina, the ridge which 

 separates the area from the lanceolate space ; the median carina is a ridge or line of 

 tubercles which passes longitudinally along the middle of the area. 



Triuonia subglobosa. Tab. V, fig. 21. 



Testa suborbiculari, convexd, umbonibus prominulis recurvatis; margine anteriore et 

 inferiore rotundato, margine posterior e brevi et concavo ; area brevi, lata transverse striata, 

 carinis tribus ornatis, carinis tuberculosis, carina marginali tuberculis majoribus ; costis 

 numerosis per series angulatis dispositis, postice magnis et tuberculatis, antice lavigatis 

 crebris interdum obsoletis. 



Shell subglobose, umbones prominent and recurved, the anterior and inferior margins 

 rounded, the posterior margin short and somewhat concave ; area short and wide, striated 

 transversely, and ornamented with three tuberculated carinas, of which the marginal 

 carina has the larger tubercles ; the space between the inner carina? is smooth and short ; 

 the other portion of the shell has numerous closely-arranged oblique tuberculated costse 

 which form a series of angles upon the middle of the shell, the angles being usually greater 

 than right angles. The anterior portions of the costae pass obliquely downwards to meet 

 the posterior portions, they are smooth, usually undivided, and towards the lower part of 

 the shell become nearly obsolete ; their posterior portions are large, forming irregular 

 varices, which are very prominent. It is nearly allied to Trigonia Goldfussii in the 

 character of its surface, the chief distinction consisting in the fewer costae and less acute 

 angle of the latter species ; but the figure of the two species is very different. 

 T. Goldfussii is much more flattened and less elougated posteriorly, the utnbones are not 

 recurved, the figure of the area altogether is more lengthened and straight, and it likewise 

 attains to a larger size than T. subglobosa. 



Our species occurs rarely both in the shelly beds of the Great Oolite and in the upper 

 part of the middle division in the Inferior Oolite. 



Localities. Minchinhampton Common, in the Great Oolite ; Nailsworth, or Scar Hill, 

 in the Inferior Oolite. 



