﻿QUADRATvE. 



103 



No. 1. — Length across the valve 

 Height 



Length of the escutcheon 

 Length of the siphonal border 

 Across the area 

 Across the paUial surface 



No. 2. — Length across the valve 

 Height 



Length of the escutcheon 

 Length of the siphonal border 

 Across the area 

 Across the pallial surface 



12 lines. 



Specimens of less mature growth have the length and height equal, the measure- 

 ment across the area is also equal to that across the pallial surface. 



StratigrapUcal positions and Localities. — The typical form of T. dadalea is not 

 uncommon in the lower beds of Greensand at several localities in the Blackdown 

 region, more especially in the Whetstone Pits near to Lyme Regis, Honiton, and 

 Collumpton, 



The south-western outhers of the Greensand at Great and Little Haldon have not 

 produced the typical form, excepting in its very young condition. It appears to be also 

 absent in the coast sections of Upper Greensand and Chloritic Marls upon the coast of 

 South Devon, and equally so at the Isle of Wight, iu Wiltshire, and in Kent and Sussex, 

 and as no foreign example is known it may be presumed to have been a species 

 eminently localised and restricted to a very limited area. 



The large variety confusa, Plate XXIII, fig. 1, was obtained by Mr. Vicary at Little 

 Haldon, near Dawlish, in the second bed of Greensand, which underlies the bed with 

 Orbitulites. As these latter fossils are abvuidant and do not occur at Great Haldon, 

 which is separated from the other hill only by the narrow valley of Escombe, Mr. Vicary 

 inclines to the opinion that the upper beds at Little Haldon are somewhat higher in the 

 series than the other. The pebbly stratum enclosing Greensand fossils, including 

 Trigonia i^ennata, T. sulcataria, and T. Vicaryana, with numerous other forms well 

 preserved, caps the hills both of Great and Little Haldon. These Tric/onice do not occur 

 in the Blackdown region. 



Afuiities and differences. — The only figure of T. dadalea, other than those of 

 Parkinson and of Sowerby, which has come under my notice is a variety which con- 

 stitutes the T^.^^fz/wft/aof Deshayes (Leymerie, ' Mem. Soc.Geol. Fr.,'ser. 2e,vol. v, plate viii, 

 fig. 5). Compared with British specimens, the small but distinctly raised marginal 

 carina is destitute of nodes ; the rows of pallial varices are much fewer ; small at the 



