58 BRITISH FOSSIL TRIGONI^. 



variety have the costse, both anteal and posteal, plain ; the posteal series, hov4^ever, are 

 usually slightly nodulous. 



T. paucicosta is nearly allied to T. angulata. Sow., but has a much shorter anteal, 

 and more lengthened posteal side; the ornamentation of the area is much larger, 

 especially the tubercles upon the marginal carina, the few larger nodes upon the posteal 

 series of varices are different to the small tubercular rows of T. angulata ; the angles 

 which the rows form at the middle of the valves are also distinct from the curvature or 

 undulation of T. angulata. Another allied form is T. undulata, Fromherz (Agassiz, 

 'Trigonies,' pi. x, fig. 4), also a specimen figm-ed under the same name (pi. vi, fig. 1) ; 

 the description refers only to the former of the two specimens, which has the varices, 

 both anteal and posteal, equal in size, and united mesially ; the marginal carina has 

 unusual prominence. The specimen, pi. vi, fig. 1, may possibly be identical with our 

 T. paucicosia, but it appears to have undergone compression, and is therefore scarcely to 

 be relied upon ; should its identity with oar species be eventually established, the name 

 I have chosen may remain, as it is sufficiently distinct from the typical form figured by 

 Agassiz, which may be regarded as the true T. undulata ('Trig.,' pi. 10, fig. 4). 



For comparison with T.flecta, another allied species, the reader is referred to that 

 shell. 



The specimens selected for our figures sufficiently exemplify the general variability of 

 the species, and also the changes of aspect produced by advance of growth ; usually 

 examples of very advanced growth are more imperfectly preserved, and are therefore less 

 fitted to exemplify the species. 



There is so much variability, both in the figure and ornamentation, that measure- 

 ments of proportions have but little utility, descriptions also must, to some extent, be 

 subordinate to figures in conveying correct or sufficient ideas of its several aspects. 



Stratigraphical position and Localities. Hitherto T. paucicosta has been obtained 

 only in the Kelloway Rock, of Cayton Bay, three miles to the southward of Scarborough. 

 Numerous specimens in various stages of growth occur in the higher beds of the 

 formation at that locality, in hard, brownish, sub-siliceous rock, occupying a thickness of 

 eight feet, and associated with a multitude of the characteristic Ammonites of the 

 formation. Other beds, six feet or more, separate it from certain lower hard beds, 

 mostly of pale grey colour, which yield Trigonia Bupellensis in considerable numbers, 

 even down to the dark clay which separates it from the Cornbrash. (The discovery of 

 numerous examples of the latter species having occurred since page 28 was printed, 

 I take the present opportunity of mentioning that it is intended to give additional 

 illustrations of it upon the last plate of the present Monograph.) The entire thickness 

 of Kelloway Rock at Cayton Bay, and beneath the adjacent Red Cliff does not exceed 

 twenty-five feet, the section of the same formation to the northward of Scarborough 

 Castle is upwards of three times that thickness, including a portion of the highest 

 beds removed for foundations of houses. These excavations produced the finest 



