California is separated from these islands by a third of the circumference of the globe, one-half of the interval being occupied by the broad North Atlantic ocean. The fresh waters of California, however, contain crayfishes which are {244} so like our own, that it is necessary to compare the two in every point mentioned in the foregoing description in order to estimate the value of the differences which they present. Thus, to take one of the kinds of crayfishes found in California, which has been called Astacus nigrescens; the general structure of the animal may be described in precisely the same terms as those used for the English crayfish. Even the branchiæ present no important difference, except that the rudimentary pleurobranchiæ are rather more conspicuous; and that there is a third small one, in front of the two which correspond with those possessed by the English crayfish.
The Californian crayfish is larger and somewhat differently coloured, the undersides of the forceps particularly presenting a red hue. The limbs, and especially the forceps of the males, are relatively longer; the chelæ of the forceps have more slender proportions; the areola is narrower relatively to the transverse diameter of the carapace (fig. [61], C). More definite distinctions are to be found in the rostrum, which is almost parallel-sided for two-thirds of its length, then gives off two strong lateral spines and suddenly narrows to its apex. Behind these spines, the raised lateral edges of the rostrum present five or six other spines which diminish in size from before backwards. The postorbital spine is very prominent, but the ridge is represented, in front, by the base of this spine, which is slightly grooved; and behind, by a distinct spine which is not so strong as the postorbital spine. {245} There are no cervical spines, and the middle part of the cervical groove is angulated backwards instead of being transverse.
FIG. 62. A & D, Astacus torrentium; B & E, A. nobilis; C & F, A. nigrescens. A–C, 1st abdominal appendage of the male; D–F, endopodite of second appendage (× 3). a, anterior, and b, posterior rolled edge; c, d, e, corresponding parts of the appendages in each species; f, rolled plate of endopodite; g, terminal division of endopodite.
The abdominal pleura are narrow, equal-sided, and acutely pointed in the males (fig. [61], F)—slightly broader, more obtuse, and with the anterior edges {246} rather more convex than the posterior, in the females. The tergal surface of the telson is not divided into two parts by a suture (fig. [61], I). The anterior process of the epistoma is of a broad rhomboidal shape, and there are no distinct lateral spines.
The squame of the antenna is not so broad relatively to its length; its inner edge is less convex, and its outer edge is slightly concave; the outer basal angle is sharp but not produced into a spine. The opposed edges of the fixed and movable claws of the chelæ of the forceps are almost straight and present no conspicuous tubercles. In the males, the forceps are vastly larger than in the females, and the two claws of the chelæ are bowed out, so that a wide interval is left when their apices are applied together; in the females, the claws are straight and the edges fit together, leaving no interval. Both the upper and the under surfaces of the claws are almost smooth. The median ridge of the endopodite of the sixth abdominal appendage is more marked, and ends close to the margin in a small prominent spine.
In the females, the posterior division of the sternum of the penultimate thoracic somite is prominent and deeply bilobed; and there are some small differences in form in the abdominal appendages of the males. More especially, the rolled inner process of the endopodite of the second appendage (fig. [62] F, f) is disposed very obliquely, and its open mouth is on a level with the base of the jointed part of the endopodite (g) instead of reaching almost to {247} the free end of the latter and being nearly parallel with it. In the first appendage (C), the anterior rolled edge (a) more closely embraces the posterior (b), and the groove is more completely converted into a tube.
It will be observed that the differences between the English and the Californian crayfishes amount to exceedingly little; but, on the assumption that these differences are constant, and that no transitional forms between the English and the Californian crayfishes are to be met with, the individuals which present the characteristic peculiarities of the latter are said to form a distinct species, Astacus nigrescens; and the definition of that species is, like that of the English species, a morphological abstraction, embodying an account of the plan of that species, so far as it is distinct from that of other crayfishes.