184 PERMIAN FOSSILS. 



which is hollowed out on its free side, perpendicularly sulcated on its anterior and 

 posterior sides, and which fits exactly into the triangular space between the two 

 divaricating ridges of the right valve {JB — c*). On each side of the central triangular 

 projection there are two deep furrows {J — b, d), which respectively receive each of the 

 two large ridges of the right valve : the anterior one is bordered in front by a small 

 ridge [A — a), sulcated on its posterior side, and corresponding to the small furrow in 

 advance of the anterior large ridge of the right valve [B — a*) ; and the posterior one 

 is bordered behind with a similar small ridge {A — e), sulcated on its anterior side, and 

 corresponding to the small furrow behind the posterior large ridge of the right valve 

 [B — e*). It will thus be evident, that the large central triangular projection of the 

 left valve, and the two small ridges lateral to it, are three teeth, which respectively fit 

 into the triangular space, and its two lateral furrows, of the right valve ; and that, on the 

 other hand, the two large ridges of the latter valve are also teeth, which fit into the two 

 large furrows of the opposite one. It is stated by Lamarck and others, that there are 

 two teeth in the right valve, and four in the left one, in Trigonia ; but it is not correct 

 to consider the large central triangular projection of the left valve as constituting two 

 teeth ; since it is clearly only one, which is dilated and hollowed out, or cleft, as it 

 were, on its free side, so as to appear like two teeth joined at the base.^ But, however 

 different the parts of the hinge, properly so called, may be in the two valves, it is not 

 the same with the .rounded ridge which is prolonged beneath the anterior part of the 

 hinge of the left valve, as it is perfectly conformable to that of the right valve ; neither 

 is it so with the impressions of the anterior adductor muscle, or of the posterior adductor 

 and supplementary (pedal or visceral) muscles, which are perfectly symmetrical in both 

 valves. The pallial impression extends from the anterior to the posterior adductor 

 muscular impressions, without showing any trace of sinuosity.^ 



M. Agassiz includes in the present family the genera Trigonia and Mgojjhoria : to 

 these I propose adding the genus Schizodus. Myojjhoria and Schizodus appear to have 

 been the earliest created forms of the family, as they are found in the Carboniferous, 

 Permian, and Triassic deposits ; whereas Trigonia apparently does not occur in any 

 formations earlier than the Jurassic :^ it has also been found in nearly all the subse- 

 quently-formed deposits, and is still an inhabitant of the present seas, though exceed- 

 ingly limited both as regards localities and species.* 



^ The following is Lamarck's description of the dental s-^siem. oi Trigonia : " teeth cardinal, oblong, 

 flattened on the sides, diverging, sulcated transversely : two in the right valve, and sulcated on both sides : 

 four in the opposite valve, and sulcated only on one side." (Animaux sans Vert^bres, 2d Ed., t, vi, p. 512.) 



2 The paragraph with which this note is connected, is partly a verbatim copy, and partly an abridgment 

 of Agassiz' description contained in his Memoir on the family Trigoniidce. 



3 I have an impression, that M, Alcide d'Orbigny has described a South American species of Trigonia, 

 which he considers to be carboniferous, 



* When Agassiz wrote his Memoir, no species of Trigonia had been found in deposits newer than the 

 secondary ; but since then, tertiary species have been found in Australia ; and, if my memory does not fail 

 me, in South America, by M. A. d'Orbigny. 



