The length of the rostrum, measured from the orbit {232} to its extremity, is greater than half the distance from the orbit to the cervical groove. It is trihedral in section, and its free end is slightly curved upwards (fig. 41). It gradually becomes narrower for about three-fourths of its whole length. At this point it has rather less than half the width which it has at its base (fig. 61, A); and its raised, granular and sometimes distinctly serrated margins are produced into two obliquely directed spines, one on each side. Beyond these, the rostrum rapidly narrows to a fine point; and this part of the rostrum is equal in length to the width between the two spines. The tergal surface of the rostrum is flattened and slightly excavated from side to side, except in its anterior half, where it presents a granular or finely serrated median ridge, which gradually passes into a low elevation in the posterior half, and, as such, may generally be traced on to the cephalic region of the carapace. The inclined sides of the rostrum meet ventrally in a sharp edge, convex from before backwards; the posterior half of this edge gives rise to a small, usually bifurcated, spine, which descends between the eye-stalks (fig. 41). The raised and granulated lateral margins of the rostrum are continued back on to the carapace for a short distance, as two linear ridges (fig. 61, A). Parallel with each of these ridges, and close to it, there is another longitudinal elevation (_a, b_), the anterior end of which is raised into a prominent spine (_a_), which is situated immediately behind the orbit, and may, therefore, be termed the _post-orbital_ {233} _spine_. The elevation itself may be distinguished as the _post-orbital ridge_. The flattened surface of this ridge is marked by a longitudinal depression or groove. The {234} posterior end of the ridge passes into a somewhat broader and less marked elevation, the hinder end of which turns inwards, and then comes to an end at a point midway between the orbit and the cervical groove. Generally this hinder elevation appears like a mere continuation of the post-orbital ridge; but, sometimes, the two are separated by a distinct depression. I have never seen any prominent spine upon the posterior elevation, though it is sometimes minutely spinulose. The post-orbital ridges of each side, viewed together, give rise to a characteristic lyrate mark upon the cephalic region of the carapace. [Illustration: FIG. 61.—A, D, & G, _Astacus torrentium_; B, E, & H, _A. nobilis_; C, F, & I, _A. nigrescens_ (nat. size). A–C, Dorsal views of carapace; D–F, side views of third abdominal somites; G–I, Dorsal views of telson. _a, b_, post-orbital ridge and spines; _c_, branchio-cardiac grooves inclosing the areola.]