It is at present an open question whether further investigation will or will not bear out either of these assumptions. If large series of specimens of both stone crayfishes and noble crayfishes from different localities are carefully examined, they will be found to present great variations in size and colour, in the tuberculation of the carapace and limbs, and in the absolute and relative sizes of the forceps.
The most constant characters of the stone crayfish are:—
1. The tapering form of the rostrum and the approximation of the lateral spines to its point; the distance between these spines being about equal to their distance from the apex of the rostrum (fig. [61], A).
2. The development of one or two spines from the ventral margin of the rostrum.
3. The gradual subsidence of the posterior part of {295} the post-orbital ridge, and the absence of spines on its surface.
4. The large relative size of the posterior division of the telson (G).
On the contrary, in the noble crayfish:—
1. The sides of the posterior two-thirds of the rostrum are nearly parallel, and the lateral spines are fully a third of the length of the rostrum from its point; the distance between them being much less than their distance from the apex of the rostrum (B).
2. No spine is developed from the ventral margin of the rostrum.
3. The posterior part of the post-orbital ridge is a more or less distinct, sometimes spinous elevation.