﻿CLAVELLAT^. 53 



Trigonia Williamsoni, Lycett, sp. nov. Plate XVI, fig. 8. 



Shell ovately oblong, lengthened, depressed ; umbones sub-anterior, not much elevated, 

 obtuse, not reciu'ved ; anterior side short, its border somewhat truncated, curved 

 elliptically at its base with the lengthened lower border ; superior border nearly straight, 

 or slightly convex, sloping downwards, its extremity rounded with the posteal termination 

 of the lower border. Area narrow and flattened, bounded by two faintly traced, minutely 

 tuberculated carinas ; there is also a similar, rather obscure, line of tubercles indicating 

 the position of a median carina ; there are also delicately marked, transverse, plications 

 of growth, which become more prominent posteally. The escutcheon is flattened or 

 slightly excavated, and has great length ; it is narrow, in conformity with the area. The 

 other portion of the valve has about ten or eleven oblique, or slightly curved rows of large 

 depressed, nodose varices ; the first-formed three or four rows form separate nodes, but 

 with some irregularity and inequality in their arrangement ; the succeeding rows have 

 the nodes very large, confluent, and depressed near to the angle of the valve, becoming 

 rapidly small, attenuated, and irregular, near to the pallial border. The lines of growth 

 are strongly defined over the whole of the valve. 



This is one of the most depressed forms of the Clavellata ; it possesses some general 

 resemblance to T. triquetra, but is more depressed, and more lengthened ; the posteal por- 

 tion is wider and more rounded ; its rows of nodose varices are also much more oblique. 

 From T. davellata it is distinguished by the very short anterior side, by the nar- 

 rower area and escutcheon, by the general depression of the valves, and by the few broad, 

 irregular, confluent nodes at the carinal extremities of the rows. 



The name is intended as a trifling tribute to reminiscences of the earlier geological re- 

 searches of Professor W. C. WiUiamson, F.R.S., of Owen's College, Manchester, and of 

 his description of the locality whence the examples of this Trigonia have been obtained.^ 



Stratic/rcqjhical position and Localities. The Kelloway Rock, of Cayton Bay, near 

 Scarborough. The matrix is a very hard, variable, grey, or sometimes whitish, siliceous 

 rock in the lower portion of that stage ; the valves of conchifera occm- in abundance, but 

 owing to the very intractable kind of rock, few are separated in good condition ; associated 

 with it are numerous valves of Trigonia Rupellensis. T. IFilliamsoni appears to be rare, 

 only two examples have come under my notice ; it is intended to figure a more perfect 

 specimen upon the last plate of this Monograph. 



1 "On the Distribution of Organic Remains upon the Yorkshire Coast." 'Trans. Geol. See.,' 2 ser., 

 vol. vi, p. 143; 1838. 



