INTRODUCTION. 3 



so far as regards the species of the Cretaceous System ; but in the Jurassic System his 

 stratigraphical distribution of them is full of errors, and is, perhaps, more calculated to 

 mislead than to instruct. These critical, and it is hoped not unfair, remarks are not how- 

 ever, intended to apply to the descriptions of Trigoniae to be found in various memoirs 

 and monographs which date within the last twelve years, referred to in the following 

 pages ; in these the requirements of modern science are more fully complied with, and 

 by authors whose names are a guarantee for the faithfulness and value of their contributions. 

 The descriptions by Dujardin, Hebert, Munier-Chalmas, de Loriol, Credner, Coquand, 

 and Pictet, are more especially prominent and satisfactory to the student in Palaeon- 

 tology. For the most part, however, each such contribution refers only to the species 

 which pertain to a single formation and locality. 



The British Jurassic Trigoniae are remarkable both for their number of species and 

 the variety in their ornamentation ; no other country has produced so considerable an 

 assemblage. The British Cretaceous species, on the other hand, although eminently 

 characteristic, represent only a portion, perhaps scarcely a moiety, of the foreign Cretaceous 

 forms, if we include those of South America and Southern Asia. 



The woodcuts illustrative of various foreign forms allied to British Trigoniae, together 

 with the concise notes which refer to them, will, it is hoped, add somewhat to the value 

 of the Monograph, and be found more satisfactory than any unaided descriptions, 

 however copious they may be. In the order of arrangement each species is placed in 

 its sectional position, adjacent to its allied forms, and at the end of the Monograph a 

 general table will be given indicating the stratigraphical position of each species. The 

 general description of sectional characters which follows conduces to some abbreviation, 

 and obviates the necessity for repetition in the description of each separate species, so 

 far as these sectional characters apply. There are also a few terms employed by which a 

 single word or two suffices to indicate the portion of the shell referred to, and, therefore, 

 materially aids in the same result. Throughout all the fossil species of Trigoniae there 

 are certain features connected with the general figure and ornamentation which, although 

 only of subordinate importance when considered in relation to the generic characters, 

 nevertheless furnish valuable aids both in the separation of the several groups into which 

 the genus may be divided, but also afford guides to the determination of the strati- 

 graphical position which they occupy. The living Australian species are in their external 

 characters to a great extent unconnected with these distinctive features ; they assimilate 

 more nearly to other generic forms of the Conchifera, and therefore constitute a group 

 apart and disconnected from the chain of fossil Trigoniae of the various Secondary 

 Formations, between which and the living species the missing connecting links will 

 probably only be discovered when other examples of the genus shall be procured in some 

 one of the Lower or Middle Tertiary Formations. 



The surface of every fossil Trigonia is clearly divided into two portions ; one of these 

 occupies the anteal, the other the posteal portion of the valve; the two parts have 



