﻿UNDULATE. 71 



Compared with T. conjunc/ens, its ornamentation is much less prominent ; its posteal 

 varices are larger and more depressed ; the area is less expanded posteally, and is destitute 

 of the large rugose plications which are so conspicuous on that species. 



Localiiy. The finely laminated, slaty sandstone has preserved the ornamentation in a 

 very perfect manner, although the test has wholly disappeared. It has only been recognised 

 in the vicinity of Collyweston, and in an apparently similar geological position in the 

 Inferior Oolite of Northamptonshire. 



Trigonia Leckenbyi, Lycett, sp. nov. Plate XVI, figs. 1, 2. 



Shell ovately oblong, lengthened and attenuated posteally, much depressed ; umbones 

 sub-anterior, obtuse, not conspicuous ; anterior side short and rounded ; lower border 

 lengthened, curved elliptically ; superior border lengthened, slightly concave, and having 

 a gentle curvature downwards posteally to the lower extremity of the area, which is 

 somewhat pointed. Area narrow and flattened with some obscure transverse plications ; 

 the bounding carinas are scarcely elevated, the marginal carina has a row of small tubercles, 

 which disappear posteally ; there is no distinct mesial furrow. The escutcheon is 

 lengthened, very narrow, and depressed. The other portion of the valve has the rows of 

 costas in two series ; the posteal series consists of depressed, curved, rounded, sub- 

 tuberculated costae, which pass downwards almost perpendicularly from the carina to the 

 middle of the valve ; their number is from fourteen to sixteen, they are cord-Uke, and 

 become attenuated near to the carina; the anteal series is much smaller and more 

 numerous, the rows are sub-tuberculated, and, for the most part, nearly horizontal in their 

 direction ; they are not distinctly united to the extremities of the posteal series, but 

 become broken into small, irregular, isolated tubercles, which occupy the middle of the 

 valve, even to the lower border ; the general direction of the rows of posteal costse is 

 therefore not conformable with that of the anteal series. The lines of growth appear to 

 have but little prominence ; no portion of the test has been preserved in the larger 

 example, and only partially so in the smaller. 



This remarkable example of the Undidata, conspicuous for its very much depressed, 

 lengthened figure, and the delicate, but composite, character of its ornamentation, possesses 

 some general resemblance to T. angulata, from which it will readily be distinguished by its 

 large obtuse umbo and depressed figure ; or, when the costse are preserved, the very 

 numerous and minutely knotted anteal series is a characteristic feature. The lengthened 

 depressed form equally removes it from T. literata, Phil., to which it approximates in the 

 general character of its costae. Other species more remotely allied are T. v. -cost at a and 

 T. jjaucicosta, to the descriptions of which the reader is referred. T. Bamsayi, from 

 nearly the same geological horizon in Gloucestershire, with a more lengthened form and 



