Lamellibranehiate shells are the most abundant fossils of most rocks. Numerous in genera, and prolific in species, they are multitudinous in individuals, and the specimens vary. The study is not easy. It has little of the poetry of many other branches of natural history, and has naturally received less attention. But in each subkingdom specially, no less than in the animal kingdom generally, the law holds good that the lower the organization the longer is the duration in time. And so, important as the higher mollusca are in the analysis of rocks into their several zones, it is on the lower forms we rely in synthetical arrangements. The Conchifera readily divide rocks into Palaeozoic and Neozoic,—no form with a pallial sinus being known below the Lower Secondary strata; while, from the appearance in them of numerous new genera, no class of animals better marks the recognized systems into which fossiliferous rocks are grouped. It is indispensable both to the geologist and biologist to be familiar with the genera, and it is the object of this paper to render the principles on which they are identified more easy and exact.

That genera are practically realities every student knows well; as such the geologist and zoologist have to do with them. And it has elsewhere been shown that between hosts of groups intermediate forms can no more be found, than can intermediate wood fill the space between the forked branches of a bush. Accepting the fact, the question arises,—How may genera be known?