﻿ADDENDA. 



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The larger of the two specimens has the costse somewhat more oblique, but does not 

 appear to differ as a species from the smaller specimen. The position and locality is the 

 Lower Calcareous Grit of Snainton, near Scarborough. 



Trigonia Rupellensis, IX Orb. Page 28, Plate VIII, fig. 4 ; also Plate XXXVI, figs. 

 1, 2, 3, 4. 



During many years the original of our figure PI. VIII, fig. 4, was the only well- 

 preserved British specimen known. Recently a considerable number, representing every 

 stage of growth and varying greatly in figure and in surface ornaments, have been 

 procured in shore beds of Kelloway Rock at Cayton Bay, near Scarborough. The rock, 

 hard, subsiliceous, varying in colour and structure, has been found to contain over a 

 small area a profusion of these Trigonia, with both separated and united valves ; but, 

 owing to the intractable and tough matrix, only a small minority have been obtained in 

 a condition suitable for the cabinet. The additional figures, on PI. XXXVI, exhibit 

 considerable differences both in the figure of the shell and in the surface ornaments. 



In some instances, as in PL VIII, fig. 4, the form is ovately trigonal and short 

 posteally; the costte are curved, and have not much general irregularity ; more frequently 

 the figure is ovately oblong, lengthened posteally ; the costae, or some of them, are broken 

 mesially, angulated, directed anteally ; or in other examples the rows of costse descend 

 obliquely in a confused and irregular manner to the pallial border. The surface 

 ornaments generally have so much irregularity that scarcely any two specimens fully 

 developed present any near approximation in their general aspect. The nodes in the 

 rows also partake of the general variability ; usually the larger nodes are those near to 

 the marginal carina ; they are rounded, obtuse, and depressed ; the smaller nodes are 

 more compressed, pointed, and elevated. 



Specimens representing the earlier stages of growth have but little of the variability 

 exhibited by more adult forms ; they might readily be mistaken for young examples of 

 other clavellated species, and have therefore not been figured upon our plates. 



Numerous specimens have suffered compression, or have their tests only partially 

 preserved ; a few examples have the valves in contact, and in such the internal moulds 

 have been more or less exposed. The cardinal processes are small, the hinge-border is 

 concave, the posteal portion is depressed, the borders are rounded ; the test is thin ; 

 the lines of growth are large, uniform, and conspicuous whenever the surface is well 

 preserved. 



Notwithstanding the considerable differences of figure and of surface ornaments, it 

 does not seem possible to arrange them as distinct varieties ; the additional figures 

 illustrate sufficiently the several aspects of this species. 



