173 



shells, of different genera, being combined in this one shell : " Une 

 espece de coquilles des plus rares dans le regne des fossiles, qu'on ne sait 

 encore sous quel genre elle pourroit etre convenablement rangee, vu 

 qu'elle tient en meme terns, beaucoup du musculite, de Parche et de la 

 conque de Venus. Elle approche fort du musculite par le raport de sa 

 longueur a la largeur ; elle ressemble a un arche, par une petite emi- 

 nence, ou une espece de petite plate forme qui se trouve entre les deux 

 bords de la charniere : et les bords, qu'elle a un peu convexes d'un cote, 

 lui donne la ressemblance avec la conque de Venus." Monument des 

 Catast. Tome n. p. 66. 



Notwithstanding the combination of the characters of the muscle, ark, 

 and Venus, as observed by Walch, and to which may be added those 

 of the cockle and the tellen, these shells vary so much from every other, 

 as to render their description difficult. On first view, the shell appears 

 reversed : the anterior end, on which, particularly in the Veriuses, the 

 area surrounding the cartilaginal depression k disposed, and which is in 

 general flat, as if it were truncated, is, in these shells, considerably ex- 

 tended out; whilst the beaks of the shell are turned towards the anterior 

 side, leaving the posterior side with no cordiform impression, but having 

 all the appearance of the anterior side, in the shells of the Venus kind. 



Bruguiere has, I understand, described, in V Encyclopedic Methodique, 

 four species of this genus ; but, being unable to obtain this work, I am 

 uninformed with which of the species he is acquainted. 



The first information which I gained respecting the generic character 

 of these shells, was from a fossil purchased from Mr. Strange's collec- 

 tion ; in which the left had so slipped from the right valve, as very fully 

 to display the structure of the hinge. Bruguiere having been so fortu- 

 nate, as, by clearing a valve, to discover the kind of hinge which it pos- 

 sessed, found it necessary to form a genus for the reception of these 

 shells, and named it Trigonia, from the form which generally belonged 

 to the species then known. 



But, as in my specimen, so in Bruguiere's, it was the hinge part of 



