﻿12S BRITISH FOSSIL TRIGONI^E. 



all the costse are more or less slightly crenulatcd, the posteal series have their edges 

 acute, rough, and irregularly scabrous ; their lower extremities alternate at the border 

 with those of the other valve, producing a pecviliarly jagged outline. Upon well- 

 preserved examples the anterior face of the valves discloses a considerable number of 

 minutely knotted radiating lines, which cross the wide intercostal spaces and become 

 evanescent near the lower curvature of the valves ; they appear to be irregular in their 

 distribution and remind us of the surfaces of the Areas. The less wide interstitial spaces 

 of the posteal costoe have occasionally each a single mesial perpendicular line, but more 

 frequently these are evanescent. The escutcheon also exhibits some traces of radiating 

 lines similar to those upon the anteal face of the shell. Young specimens do not differ 

 from adult forms in any material degree ; they are, therefore, equally distinctive when 

 compared with contemporaneous species. 



This large and remarkable species of the ScahrcB has been mistaken for T. .caudata, 

 and tabulated as such in lists of Isle of Wight fossils. Compared with T. caudata, our 

 species is much larger, the umbones are much more elevated and larger, the anterior side 

 is more short and flattened, the caudal extremity is shorter ; the upper border of the 

 escutcheon is much more concave or depressed, its costellaj do not form fringing 

 prominences as in T. caudata ; the costaa are narrower, more elevated and acute ; they 

 are destitute of obtuse, crenulated, fringing outer borders ; the presence of the inflated 

 lobes at the anteal flexure of the valves upon four of the costae, and of radiating knotted 

 lines upon the anterior flattened surface, gives also distinctive features. The peculiarly 

 narrow and scarcely defiued area, destitute of distinct costellse, also affords a contrast to 

 the prominent area and its well-marked costellse in T. caudata. From T. aliformis it is 

 distinguished by the few, widely separated, acute, perpendicular posteal costaj, by 

 the absence of the sudden inflation which characterises the anteal half of the valves, 

 by the wide, compressed anteal surface, by the greater breadth of the united valves, and 

 by the few large strictly transverse costellse upon the escutcheon, contrasted with the very 

 numerous, delicate, oblique, and almost evanescent costellse of T. aliformis; generally, 

 also, by the wide intercostal spaces and by the radiating knotted lines upon the flattened 

 anterior surface in well-preserved specimens. The lower border is also remarkably 

 distinct. T. aliformis has the extremities of the perpendicular costse upon each valve, 

 corresponding each to that of the opposite valve. In T. Etheridgei they alternate, thus 

 producing a peculiarly jagged outline at the border. 



Dimensions of an adult sjjecimen. — Height 2| inches; length, horizontally, 3 inches; 

 diameter through the united valves anteally, 2| inches. 



StratigrapJiical position and Localities. — In the Isle of Wight it occurs only in the 

 lowest or Perna Mulleti beds of the Atherfield clay. The valves are usually united, and, 

 owing to the large size and inflated figure, it is one of the most remarkable species of that 

 peculiarly rich assemblage of Neoconiian fossils. The test is thick, and specimens are 

 usually well preserved ; it occurs in some abundance. It will be observed that this is a 



