the front. Schmidt [1883, p. 71] described it as a subquadrangular plate with convex anterior and concave posterior margin, lying directly in front of the mouth and a part of the doublure of the carapace, separated from the remainder by two sutures. Laurie [1893, p. 516] has shown that the scale markings on the epistoma have their convex side directed forward contrary to the almost universal rule among eurypterids and that this fact would seem to indicate that we have here a portion of the carapace bent over. At the same time he remarks that some of his specimens are fractured along quite different lines than those Schmidt observed. One of these he reproduces on plate 2, figure 10. On another [his pl. 1, fig. 4] the "epistoma" of Slimonia is reproduced. This, however, is but the ventral marginal plate of the cephalothorax that occupies the space between the doublure of the carapace and the thin membrane surrounding the coxae and which is termed the "Randschild" by Holm. This plate in Eurypterus had been figured by Hall [pl. 80A, fig. 12] as "the lower surface of one side of the cephalic shield" and has been more fully described above. We reproduce in plate 74, figure 3 a well preserved upper lip or epistoma of P. macrophthalmus.
The term endostoma has been applied by Holm [op. cit. p. 28] to a small plate that bounds the posterior portion of the mouth. It is here figured from Pterygotus buffaloensis [plate 81, figure 4.]
It corresponds to the promesosternite of Limulus or of the scorpion group. To its anterior edge a thinner membrane is attached which passes inward in the direction of the throat and forms, therefore, the lower lip.
The metastoma or postoral plate is a highly characteristic organ of the eurypterids. It is large, somewhat variable in form but all its variations are derivable from the oval form seen in Eurypterus. Its size corresponds to the longitudinal extension of the last pair of coxae since it covers their inner margins and the interspace between them. Its frontal margin is always more or less emarginate, its margin bent under into a broad doublure connected with the membrane covering the interspaces of the ventral side. Holm has shown that there are traces still present of a
