48 BRITISH FOSSIL TRIGONE. 



lower border ; hinge border lengthened and nearly straight, sloping obliquely, and 

 forming an obtuse angle with the posteal extremity of the area ; escutcheon depressed, 

 moderately lengthened, its upper border raised ; area narrow, flattened, transversely 

 plicated ; the plications are usually large and irregular, there is a faintly marked, mesial 

 oblique furrow, and a minutely tuberculated median carina which disappears towards the 

 posteal extremity, the marginal and inner carinse are well marked ; the inner carina has 

 elevated squamous tubercles. The marginal carina is at first a narrow elevated plain 

 ridge, which soon changes to transversely compressed elevated tubercles, from which 

 originate the transverse plications upon the area ; the carina ultimately degenerates into 

 these plications. The other portion of the valve has about fourteen or fifteen rows of 

 clavellated but always elevated and sometimes sub tuberculated costas ; the first three of 

 these are plain, acute, and concentric, they are interrupted by the marginal carina, but 

 pass unchanged in character across the area; the succeeding costae are subtuberculated, 

 and pass obliquely downwards with little curvature, both anteally and posteally, the 

 posteal portions are much the larger and more distinctly clavellated, their junction with 

 the anteal portions form so many undulations or angles about the middle of the valve, 

 and the anteal series has one, two, or even more rows than the other series, so that the 

 rows are sometimes rather crowded anteally ; all are, however, tuberculated or sub- 

 tuberculated. 



The Trigonia which most nearly approximates to our species is T. arata, one of the 

 undulates from the upper zone of the Great Oolite, to which the reader is referred. T. 

 Moretoni varies much in its length, as will be seen from our illustrations ; occasionally it 

 exhibits a crowding of the narrow anteal portions of the costse, which imparts an apparent 

 confusion to the ornamentation of the shell. 



Our figures exhibit much of the variability which occurs both in the proportions of 

 the general figure and the ornamentation ; not uncommonly the costse are nearly plain, or 

 are only very slightly knotted. Neither of these several variable features appears to be 

 special to any locality or stratum. 



Si 'rati graphical positions and localities. T. Moretoni appears to range throughout the 

 whole of the Great' Oolite formation, it occurs commonly in all the shelly beds of the 

 Minchinhampton district, and attains to its full dimensions, but the far greater number 

 are immature forms from ten to fifteen lines in length. Mr. Whiteaves has obtained it 

 in Oxfordshire, Mr. S. Sharp in Northamptonshire, the officers of the Geological Survey 

 in the South Lincolnshire district, and in North Lincolnshire the upper zone of the Great 

 Oolite has produced a profusion of beautifully preserved specimens, which are the 

 originals of our figures ; for these I am indebted to the liberality of the Rev. J. E. Cross, 

 of Appleby, who has assiduously developed the fossil fauna of a district rich in the 

 testacea of the Lower Oolites. 



Prance, Bas-Boulonnais. See examples in the British Museum. 



