﻿QUADRATiE. 113 



specimens there is a space posteal to the varices which is occupied solely by prominent 

 rugose folds of growth.. 



Internally the margins of the valves are smooth, excepting the pallial pits and 

 eminences, which are small and only two or three in number. The prominences within 

 the border of the escutcheon are also only two or three, and rather obscure. The cardinal 

 teeth are large and diverge widely. All the specimens examined are single valves. The 

 internal mould has not been ascertained. 



Dimensions. — The largest example figured upon Plate XXVI is surpassed in size by 

 specimens in both of the National Metropolitan Geological Museums ; as, however, its 

 condition of preservation is good, and as it sufficiently represents an advanced stage of 

 growth, it may be accepted as a good illustration of the species. The length transversely 

 is 25 fines ; the height 34 lines ; the length of the siphonal border and of the escutcheon 

 are nearly equal, or 24 lines ; the thickness through the single valve 5 lines. A smaller 

 example has the length and height equal, but usually the length exceeds the height. 



Comparisons. — No example of the Quadratce, either British or foreign, appears to 

 possess any near affiaity with T. spedabilis in the combination of its few, simple, leading 

 features, viz. the short, depressed, subquadrate figure, the few unusually large, nodose, 

 unequal, but sometimes connected or cord-like pallial varices, together with the smaller 

 and for the most part few and irregular nodes upon the expanded area. Our smallest 

 figure, which illustrates the very young condition, is also well characterised by its high- 

 ridged acute costse and flattened form. Fully developed specimens have sometimes much 

 rude irregularity in their surface ornaments. These occasionally altogether disappear, 

 and the posteal half of the valve is then occupied solely by large rugose plications of 

 growth. Our figures are truthful and characteristic, but do not fully exemphfy these 

 conditions, which may be better appreciated by examining the series of specimens upon 

 the tablets in the British Museum. Pictet and Renevier, in their work above cited, have 

 given a good figure of the present species under the designation of T. nodosa ; no other 

 author after Sowerby appears to have figured it. D'Orbigny united it to his T. rudis, 

 together with T. nodosa, T. cinda, and 1. palmata (' Pal. Fran., Terr. Cret.,' vol. iii, p. 

 137) ; but his illustrative figures represent Neocomian forms only, and are altogether 

 distinct from T. spedabilis. 



Stratigraphical position and Localities. — It is associated in the Greensand of Black- 

 down with the other Trigonice of those beds, but, owing to the fragility of the test, 

 entire valves are rare. I am indebted to the liberality of Mr. Vicary, F.G.S., of Exeter, for 

 the gift of specimens to illustrate T. spedabilis in the present Monograph ; the species is 

 well exemplified in both of the National Metropohtan Museums ; more frequently private 

 collections have it only in fragments. 



