130 



culated costse, which are different upon the umbones, forming a 

 densely-arranged linear series, which pass horizontally across the 

 whole of the valve uninterruptedly. The costse (about twenty- 

 four in number) are narrow, closely arranged, curved and some- 

 what attenuated near the pallial border. The arrangement of 

 the rows is so close that it is sometimes difficult to discover the 

 real direction of the lines of tubercles. The usual length is 

 twenty-two lines, height eighteen lines, diameter through the 

 united valves, fourteen lines and a half. 



This fossil passes through all three sections of the Portland 

 series, not unfrequently occuring in the Limestone, of Portland. 



g 



TRIGONIA DAMONIANA, Le Lor., plate v., fig 3. 



MONO. BEIT. Foss. TRiaoNia:, Lye., Pal. Soc., p. 88, plate xviii., fig. 3., plate 

 xix, figs. 1, a, b, plate xxi., figs. 2-5. 



Damon's Geology of Weymouth, Supp., pi. 7, figs. 

 2, 3, 1880. 



Shell subovate, lengthened obliquely, convex ; umbones large, 

 erect, very prominently and somewhat pointed, much incurved, 

 and rendered bipartite by the narrow, deep sulcation produced 

 by the apical termination of the ante-carinal space ; border of 

 the valves elliptically rounded excepting the hinge-border, 

 which is straight, and lengthened, sloping obliquely ; the 

 anterior face of the valves has a large, rounded, depressed space 

 or lunule ; the escutcheon is depressed, cordif orm and strongly 

 marked by the lines of growth; the area is narrow, slightly 

 elevated or curved, traversed transversely by irregular folds of 

 growth ; it has a well marked mesial furrow. The costated 

 portion is divided into three or four zones ; the direction of the 

 row of cost) are not conformable with the sulcations ; upon the 

 anterior face of the valve they are uninterrupted and much 

 attenuated. Compared with Trigonia giblosa the general form 

 differs considerably, being shorter transversely, the concentric 

 sulcations smaller, and the umbones more elevated. 



Obs. Trigonia Damoniana is abundant in the Limestones of the 

 Isle of Portland. 



