ANIMALS. 187 



to their sulcated character, it will be seen, that in every essential point the agreement 

 is complete : for example, referring to Plates XV, fig. 29 A, B, and XIX, fig. 8 A, B, 

 there are three teeth in the left valve {A — a, c, e), and two in the right one {B — 3*, 

 d*), of both genera. The central tooth of the left valve is bifid or hollowed out on its 

 free side {A — c), and embraced by the diverging teeth of the opposite valve {B — b^, 

 d*), or, in other words, fitting into the triangular vacancy (B — 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 Sc/nzodus 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 Trigonia, it is 

 obviously an exaggerated form of an ordinary character : it is absent in Schizodus, but 

 present in Myophoria. 



The various species of Schizodus are equivalve ; some are very inequilateral {S. 

 axiniformis — Isocardia id., Phillips) ; while others are -scarcely so, {S. Bossicus, 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 {8. 

 rotundatus — Axinus id., Brown, &c.) : the umbones are more or less prominent ; and, as 

 in some species of Cardium, Tellina, ho,., their curving is differently directed, being 

 turned towards the anterior end in 8. axiniformis, whilst in S. obscums 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-fulcrum ; in S. axi7iiformis it appears to be directed more into the 

 cavity of the shell ; and in S. Bossicus 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 8. truncatus is bifid ; but in 8. axiniformis and 8. 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, aS'. axiniformis. 



The muscular impressions, owing probably to the thinness of the valves, are not 

 generally displayed on casts ; there is one species, however {8. obscurus), in which 



^ In a species of Schizodus 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. 



