ANIMALS. | 187 
to their suleated character, it will be seen, that in every essential point the agreement 
is complete: for example, referring to Plates XV, fig. 29 4, 5, and XIX, fig. 8 4, B, 
there are three teeth in the left valve (4—a, c, e), and two in the right one (B—0*, 
d*), of both genera. The central tooth of the left valve is bifid or hollowed out on its 
free side (4—c), and embraced by the diverging teeth of the opposite valve (B—é*, 
d*), or, i other words, fitting into the triangular vacancy (&—c™) between these 
teeth; further, each of the three teeth of the left valve fits into a depression in front of 
the teeth of the opposite valve (B—a*, c*, e*). It will thus be evident, that the only 
essential differences between the teeth of Schizodus and Trigonia is in their being 
sulcated and unusually large in the one,— plain and of the ordinary size in the other. 
With regard to the ridge behind the anterior muscular impression of Zrigonia, it is 
obviously an exaggerated form of an ordinary character: it is absent in Schzzodus, but 
present in JZyophoria. 
The various species of Schizodus are equivalve; some are very inequilateral (S. 
aviniformis—Isocardia id., Phillips); while others are scarcely so, (S. Mossicus, De 
Vern., S. carbonarius—Venus id., J. de C. Sow.) In general, they are more or less 
acuminated, or obliquely truncated posteriorly; the exceptions being rounded (&. 
rotundatus— Axinus id., Brown, &c.): the umbones are more or less prominent ; and, as 
in some species of Cardium, Tellina, &c., their curving is differently directed, being 
turned towards the anterior end in S. axiniformis, whilst in S. obscurus and others, they 
curve posteriorly. The teeth appear to be subject to certain modifications : the anterior 
tooth of the right valve is generally long, erect, and curved; but the posterior one 
appears to be variable: the latter in S. truncatus, King, (the species whose dental 
_ system I have studied with the most success.) is slightly oblique to the hinge-line 
below the cartilage-fulerum ; in S. axiniformis it appears to be directed more into the 
cavity of the shell; and in S. Aossicus and S. carbonarius it is so slightly developed, or 
so little separated from the hinge-plate, as to lose all distinctiveness.’ The central 
tooth of the left valve of S. truncatus is bifid; but in S. axiniformis and S. carbonarius 
it is thick and undivided, approaching in form to the corresponding tooth of 
Myophoria Goldfussi. 
In most of the species of Schizodus the valves are thin, and ornamented, particularly 
on the anterior side, with fine raised concentric lines, remarkably equidistant in certain 
species, for example, S. aviniformis. 
The muscular impressions, owing probably to the thinness of the valves, are not 
generally displayed on casts; there is one species, however (S. odscurus), in which 
| In a species of Sehizodus found in the Glasgow carboniferous shales, the posterior tooth of the right 
valve is very much elongated, and runs nearly parallel to the hinge line. 
