226 BRITISH POSSIL TRIGONIiE. 



largest examples of that section, are remarkably small, bat the valves have internally near 

 the middle of the lower border, and external to the pallial scar, a few closely placed 

 oblong pits, apparently intended to afiford attachment to an accessory ligamental 

 appendage, enabhng the Mollusk to hold the valves closed with greater power. As these 

 pits existed at several stages of growth, it is evident that they were reproduced periodi- 

 cally, and were obliterated at each period by the growth of new shell-substance (for 

 examples of this feature see PI. XXIV). The QuadrafcB, also, had their hinge-plates 

 strengthened by additional ligamentary support at the posteal extremity of the plate, 

 which has several oblique grooves, a feature which also obtains in the ali/onuis group of 

 the Scabra, but is not seen in any other section of the genus. 



The Blackdown and Haldon Greensands are represented in Belgium by the Meule de 

 Bracquegnies, in which those beds reappear both lithologically and palaeontologically ; 

 the Triyonm are, however, for the most part distinct in species. The Belgian T. dadalea 

 differs from the well-known form of Blackdown, but is identical with our variety confusa^ 

 PI. XXIII, fig. 1, which in Britain occurs rarely, at Little Haldon, much to the 

 westward of the typical form ; it is abundant at Bracquegnies, as is also T. Elisa, one 

 of the alifornns group special to that locality (Cornet and Briart, ' Acad. Roy. de 

 Belgique,' t. xxxiv). 



The Trigonia, so abundant in the Cretaceous glauconitic sands and marls, disappear 

 suddenly and entirely with the advent of the Chalk. Apparently this change is not an 

 exceptional feature as regards the Trigonice, but is connected with a similar loss of genera 

 in other Dimyarian Conchifera, a circumstance which becomes remarkable when compared 

 with the general abundance and variety of Monomyarian forms in the same deposits ; a 

 fact which has long been observed, but which has not hitherto received a satisfactory 

 explanation. 



The only record we have of the genus Trigonia in the White Chalk consists of some 

 impressions, ill preserved, named by d'Orbigny Trigonia inornata, ' Pal. Pran. Terr. 

 Cret.,' pi. 297, from Royan, Charente Inferieure, a moderately convex, subovate shell, 

 having apparently a plain area and escutcheon, and a numerous closely arranged series of 

 obliquely curved costae (about forty) covering the other portion of the shell, passing from 

 the angle of the valve to the anteal and lower borders, but so faintly traced that the 

 entire surface appears almost devoid of ornamentation ; thus approximating or apparently 

 intermediate to, the Scabra and Glabra. 



The Cretaceous Trigonia are, for the most part, localised ; of the thirty-one species 

 yielded by the British rocks fourteen only have been identified at continental localities, 

 and limited to neighbouring countries, as France, Belgium, Switzerland, Southern 

 Germany, and the Spanish Peninsula. A considerable proportion of the French 

 Cretaceous Trigonia appear also to be special to that country. 



The Aptian beds of Spain, Province of Teruel (Coquand, ' Monogr. de I'etage 

 Aptien de I'Espagne '), appear to have been deposited in a portion of a Mediterranean 



