228 BRITISH FOSSIL TRIGONI^. 



In Britain the Cretaceoas Trigonia constitute portions of four well-marked zoological 

 assemblages, separated in stratigraphical position ; they assist materially in imparting a 

 characteristic /«e«>s to each series. Of the Middle Neocoraian Trigonice, six in number, 

 one only of the Quadratce, T. nodosa, constitutes a variety of a species which in the 

 Upper Neocomian beds is represented by other forms. 



Of the others, two belong to the Clavellata, and have a Jurassic aspect so remark- 

 able that one of them, T. ingens. Lye. (PL VIII, figs. 1, 2, 3 ; PI. XXXVI, figs. 5, 6), 

 might readily be mistaken for the T. signata of the Inferior Oolite ; the other species, 

 T. Keepingi, Lye. (PI. XXXV, figs. 1, 2), with a shorter figure, has an aspect equally 

 Jurassic. In the absence of all knowledge of their position and associated fossils they 

 would undoubtedly have been assigned to the more ancient period. The Upper Neoco- 

 mian formation is represented by seven Tngonice, of which one {T. nodusa. Sow.) is a 

 variety of the Middle Neocomian form ; a second {T. carinata, Ag.) passes upwards into 

 a higher stage ; the other five species are all Scabra, and are special to that stage. The 

 third, or stage of the Blackdown and Haldon Greensands and Gault (identical also 

 with the Belgian Meide de Bracquenies), has nine ascertained species, including two of 

 the Quadi'ata, viz. T. dadalea, Park., and T. spectabilis, Sow., two of the Glabra, 

 T. excentrica. Park., and T. Itsviuscula, Lye, and five of the Scabrm. There is also 

 an internal mould in the Red Chalk or Gault of Hunstanton not sufficiently charac- 

 terised. The fourth or highest stage, consisting of Upper Greensand and Chloritic 

 Marls, has upwards of sixteen species ; and three others have been observed by Mr. 

 Meyer in the hard rocks and marly beds of the South Devon Coast, which hitherto 

 he has not been able to add to his collection. Of the sixteen, three are varieties of forms 

 met with in the third series, as T. aliformis, T. spinosa, and T. Vicaryana ; three others 

 which have passed upwards without apparent change, as T. scabricula, T. Iceviuscula, 

 T. spectabilis, and T. carinata ; leaving ten other species apparently special to the highest 

 stage. 



D'Orbigny assigned the maximum development of the genus Trigonia to the highest 

 fossiliferous beds of the Cretaceous rocks [Cours element, de paleont., Tableau 8). In 

 Britain our stratigraphical table records the greatest number of species in the Lower 

 Mesozoic rocks or Inferior Oolite, and in the Upper Greensand and Chloritic Marls of Wilts 

 and of the Isle of Wight. The hard rocks in the cliffs of the South Devon Coast have also 

 produced Irigonia, for the most part in a very ill state of preservation ; and great difficulty 

 is experienced in procuring useful and reliable specimens. Enough, however, is ascertained 

 to assure us that the genus was represented in Britain at this the period of its final dis- 

 appearance in a manner both ample and varied. For these the reader is referred to the 

 stratigraphical table, also to the figures on Plates 23, 24, 25, 26, 28, 35, 37, 40, and 41. 



The Cretaceous Trigonia obtained in the American continent, although not numerous 

 in species, are not less remarkable and well characterised. With the exception of a 

 single lengthened species of the Glabra from Columbia {T. Lajogei, d'Orb.), all pertain to 



