128 



its surface has small closely arranged, delicate, oblique plica- 

 tions ; area slightly excavated and flattened, rendered distinctly 

 bipartite by the superior or inner half being more depressed than 

 the other portion ; it is bounded by two well defined small carinse ; 

 the marginal carina is elevated, peculiarly narrow in the left 

 valve and somewhat larger in the other. The sides of the valves 

 have a very numerous series of costae (forty or more in advanced 

 growth), they are small and somewhat unequal in size, and 

 irregular in their direction. The smallness and irregularity of 

 the costse in so large a species is a feature altogether unique in 

 the Jurassic Costatce. 



Obs. This large Trigonia has been obtained from the Cal- 

 careous Grit formation at Weymouth. 



TRIQONIA GIBBOSA, Sow., plate v., fig. 1. 



MONO. BEIT. Foss., TBJGONLE, Lyc. t Pal. Soc., p. 84, plate xviii., figs. 1-6 

 plate xix., figs. 1 a.b., 2. 



Damon's Geology of Weyroouth, Sup., pi. xvi., fig. 

 5, 1880. 



Shell somewhat inflated, subovate, or ovately oblong ; umbones 

 large, obtuse, elevated, antero-mesial and erect ; anterior and 

 inferior border elliptically curved, hinge-border concave, its 

 posteal extremity curved gently with the posteal border of the 

 area, which is narrow, slightly curved, having a mesial oblique 

 furrow ; there are no distinct bounding carinse, but near the 

 umbo the area forms a distinct angle with the more depressed 

 anti-carinal space ; the escutcheon is of moderate breadth, 

 smooth and depressed ; the costated portion occupies more than 

 half the valve; the costse in their prominence, number, and 

 general aspects possess so much variability that, without the 

 examination of numerous connecting specimens other species 

 may possibly be united to it. 



Obs. T. giblosa is limited to the Portland Oolite and Sands. 

 It is not uncommon at Portland. 



