icerae serve to suggest the ready reversibility of the pincers. On comparing the length of the pincers with that of the arm [specimen pl. 77, fig. 3] one finds that the pincers are as long as the uncontracted part of the arm. The contracted basal portion, which amounts to about one fifth of the length of the arm is, as our material and the drawings of the British specimens indicate, nearly always missing. In another specimen, this part contrasts by its thinness with the thicker test of the arm, which terminates abruptly at a convex line along the contracted part. One might at first glance infer the presence of an articulation at this point, but the continuance of the test and the contrast in thickness of it on the arm and of the basal contracted part show that this latter was rather of the nature of a membrane and probably a part of the epistoma, Its basal edge is ragged and obviously torn.
The corresponding lengths of the pincers and arms and the actual occurrence of pincers thrown back, demonstrate, we believe, the functional possibility and competency of the pincers to grasp food and carry it to the mouth. The crustaceans afford several instructive examples of analogous prehensile organs. One of these is the giant spider crab from Japan (Macrochirus kämpferi) seen now in many of the larger museums. It possesses a pair of immensely long prehensile chelate limbs which consist of two long segments and the chelae, besides a small proximal segment. In the articulation it can be doubled in the exact middle between the two equally long segments, thus serving to bring the prey readily within reach of the masticating edges of the other limbs surrounding the mouth.
In the restorations of Pterygotus, a varying number of segments has been assigned to the chelicerae. Salter and Huxley [op. cit. pl. 15, fig. 6] gave four segments (the supposed additional basal ones not visible in the dorsal view), but Woodward doubled this number in his figure [op. cit. pl. 8, fig. 1]; and the latter is retained by Schmidt [op. cit. p. 73, fig. 1B] and has long since entered the textbooks. While Salter and Huxley figure the chelicerae as rigidly straight, as they are indeed seen in our specimens,
