96 OLD RED SANDSTONE FISHES. 



could stand, so in 1892 l I reduced the number of British species of Pterichthys to 

 three as follows : 



1. Pterichthys Milleri (Ag.) with the carapace ovate inferiorly and the terminal 

 division of the arm slender and tapering. Including also Pt. latus (Ag.), comutus 

 (Ag.), quadratus (Egert.), and cancriformis (Ag.), pars. 



2. PL productus (Ag.), carapace ovate inferiorly, terminal division of arm 

 expanded externally. Including also PL cancriformis (Ag.), pars. 



3. PL oblongus (Ag.), carapace long and narrow inferiorly, terminal division of 

 arm expanded externally. 



In this paper I also expressed myself as " strongly suspecting that there is but 

 one species in the Lower 2 Old Red Sandstone of Scotland and that the distinction 

 of the arms is a sexual one." Now after a period of twelve years has passed I still 

 hold that opinion, and for the following reasons : 



1. In all the forms the external sculpture of the plates is the same, and the 

 contour of the ventral aspect of the body-carapace is liable to such an amount of 

 variation as to render it insufficient to serve as a specific character. 



2. The shape of the distal segment is, therefore, the only tangible mark of 

 distinction left, and yields us among the Pterichthys of the Scottish Orcadian beds 

 only two forms — the one PL Milleri with tapering distal arm-segment, and the 

 other PL productus, with that part peculiarly expanded. 



3. Taking into account that the taper-armed form is usually broader in its 

 contour than the broad-armed one, it seems not at all unlikely that the difference 

 is a sexual one, and that Milleri is the female and productus the male of one 

 common species. As that, however, is a position which cannot be proved, I shall 

 here describe the two forms as distinct species. 



Pterichthys Milleri, Agassiz. Plates XIX, XX, Plate XXII, figs. 1 and 2. 



1841. Pterichthys Milleri, H. Miller (ex Agassiz). Old Red Sandstone, pp. 



49 — 53, pis. i and ii. 

 1844. and Pt. latus, Agassiz, Poiss. Foss., vol. ii, pt, i, 



p. 302 (names only). 

 1844. latus, Agassiz. Poiss. Foss. v. Gres rouge, p. 12, pi. iii, 



figs. 3, 4. 

 1844. testudinarius, Agassiz. Ibid., p. 14, pi. iv, figs. 1 — 3. 



1844. Milleri, Agassiz. Ibid. 



1844. cornutus, Agassiz. Ibid., p. 17, pi. ii, figs. 1 — 4. 



1844. cancriformis, Agassiz. Pars., ibid., p. 17, pi. i, fig. 5, non 



fig. 4. 



1 ' Proc. Roy. Phys. Soc, Edinb.,' vol. xi, 1892, pp. 283—285. 



2 For " Lower " substitute now " Middle " or " Orcadian." 



