218 BRITISH FOSSIL TRIGONI/E. 



Perhaps no sectional forms are usually more clearly separated than the Glahrce and 

 the Clavellata, but certain Upper Jurassic species of the former section (see Plates 

 XVIII, XIX, XXI) occasionally acquire much of the exterior ornaments of the Clavellata ; 

 this latter section and the ScaphoidecB can sometimes only be separated rather doubtfully, 

 and a similar remark will also not unfrequently apply to the separation between the 

 Clavellatce and the Undulata. For illustrations of the TJndulata see the figures in 

 Plates XIII, XIV, XV, XVI, and XVII. Doubtless the group of the Cosfata possess the 

 most strongly defined sectional characters, more especially in the posteal slope of the 

 area and escutcheon, with the peculiarities of their carinse and costellse, together with the 

 diflFerences which they present in opposite valves of the same species ; but even these 

 features, so important in their combination, become in some instances modified or only 

 slightly defined. 



The proposed division of Trigonia into several separate genera is based upon the 

 high value attributed to the exterior form and ornaments as examples of generic character, 

 the internal and chiefly the hinge-characters being regarded as constituting features 

 pertaining to a great natural family, embracing all the groups of species. The separation 

 of the genus adopted in this Monograph is based upon the opposite principle, — that the 

 internal characters are the only features which can be relied upon as affording decided 

 distinctions more important than those of species or of subgenera, and that the modifica- 

 tions which embrace all the features connected with the external figure and surface 

 ornaments are only of subordinate or sectional value, more or less linked together, and 

 are chiefiy of interest and importance in comparing the stratigraphic value or succession 

 in geological time of these several features, and of affording separation between 

 the several series of forms of which such groups are composed. The generic distinction 

 based chiefly upon the hinge characters has the further advantage in the genus Trigonia 

 of its great convenience in legislating upon the fossil internal moulds of Conchifera, which 

 afford usually so few features supplying distinctive characters. It becomes of great 

 importance to have upon the moulds indentations of any undoubted hinge characters 

 distinct from all others, and such are supplied by the transverse sulcations upon both 

 sides of the diverging hinge-processes in Trigonia, a feature which can always be recognised 

 in well-preserved moulds and is free from ambiguity, and by which also its hinge is 

 clearly separated from that of its allied genus Myophoria. 



Upon contemplating the changes exhibited h^ \)i\& Trigonia during the long succession 

 of Molluscan life disclosed by the two great Jurassic and Cretaceous portions of the 

 Mesozoic sera, as developed both in Britain and over the continents generally, we observe 

 a long, partially broken, imperfect chain of life, even the earlier portions of which seem, 

 but as continuations of previously existing groups of allied organic forms ; of these the 

 more immediate precursor is the Mgophoria of Bronn, an important genus of small 

 Conchifers represented by numerous species, all of which are more or less allied to Trigonia 

 and one group more especially to the section of the Costata ; Myophoria is special to the 



