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 (B—c*). On each side.of the central triangular 
projection there are two deep furrows (4—4, 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 (4—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 (4—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 Zzgonza ; 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 Zrigonia and Myophoria: to 
these I propose adding the genus Schizodus. Myophoria 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 Zrigonia 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 mhabitant of the present seas, though exceed- 
ingly limited both as regards localities and species.* 
! The following is Lamarck’s description of the dental system of Trigonia: ‘teeth cardinal, oblong, 
flattened on the sides, diverging, sulcated transversely: two in the right valve, and suleated on both sides : 
four in the opposite valve, and sulcated only on one side.’”? (Animaux sans Vertébres, 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 Trigoniide. 
5 | have an impression, that M. Alcide d’Orbigny has described a South American species of Trigonia, 
which he considers to be carboniferous. 
4 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. 
_ ssaouiioadl 
