104 OLD RED SANDSTONE FISHES. 



internal rounded ridges which it bears. One of these, nearly vertical in direction 

 and gently concave behind, is coincident with the free posterior margin of the 

 plate ; the second, nearly straight, arises from near the middle of the first and passes 

 obliquely upwards and forwards to the centre of the antero-superior margin ; while 

 the third, starting from the same point behind, runs longitudinally backwards with 

 a slight upward concavity to a point somewhat below the middle of the posterior 

 margin of the anterior dorso-lateral plate. A casual inspection of the figure might 

 suggest that this line indicates the supposed suture between two original divisions 

 of the posterior ventro-lateral plate, as maintained by Jaekel (see p. 93). It is, 

 however, the impression of an internal longitudinal elevation, which happens to lie 

 directly under the course of the lateral line groove on the outer surface. 1 



Fig. 4 shows a smaller specimen from the same locality, also lying on its side, 

 but crushed quite flat. In addition to the lateral plates, it shows the anterior and 

 posterior median dorsals, and the pectoral limb with its expanded termination. 



Bemads. — As already indicated, it seems to me that Pterichthys produdus and 

 oblongus pass into each other, so the latter must be absorbed in the former, and the 

 name product )us must accordingly stand for the species. Again, between Pterichthys 

 produdus, as now defined, and Pt. Milleri there is also no thoroughgoing distinction 

 except in the form of the termination of the pectoral appendage, for the differences 

 in the proportions of the carapace are only relative. This, together with the fact 

 that the two forms always occur together in the same beds and in the same 

 localities, seems also to indicate, though it cannot be proved, that we have only one 

 species of Pterichthys in the British Orcadian rocks, and that the differences between 

 Pt. Milleri and Pt. produdus are of a sexual character. 



Geological Position and Localities. — The same as in the case of Pt. Milleri. 



Mickobkachius, Traquair, 1888. 



1867. Pterichthys, C. W. Peach (pars.). 



1888. Microbrachius, Traquair. 



1891. Microbrachium, A. 8. Woodward. 



Generic Characters. — Pectoral appendage short. Carapace broad dorsally and 

 narrow on the ventral surface. On the dorsal or upper surface the anterior margin 

 of the carapace forms a deep re-entering angle or emargination in which the head 

 is received. Antero-lateral margin of anterior dorsal plate first overlapping and 

 then overlapped by the anterior dorso-lateral ; its postero-lateral margin over- 

 lapped by the posterior dorso-lateral, as in Pterichthys and Bothriol&pis. 



1 The same internal elevations are, of course, also present in Pt. Milleri. Impressions of similar 

 ridges have also given rise in former times to the erroneous dismemberment from the anterior 

 ventro-lateral plate of a supposed " thoracic " one for bearing the arms, and of a " posterior ventral " 

 from the posterior ventro-lateral. See Sir P. Egerton on Pterichthys, ' Quart. Journ. Geol. Soc.,' 

 vol. iv, 1848, Fig. 2, p. 305, and also my remarks on p. 74, ante. 



