GEOLOGICAL SURVEY IN SILURIAN ROCKS OF SOUTH OF SCOTLAND. 841 



I thought it probable that they also occupied a ventral position in Lasanius ; after- 

 wards, however, it seemed to me more natural to suppose that such a long row of median 

 thorny plates should run along the back instead of along the belly. The matter was, 

 however, settled by a specimen of a new species of the same genus obtained by Mr Tait 

 since the beginning of the year 1899, and, in consequence, the description of Lasanius 

 has been rewritten since this paper was presented to the Society.] 



Lasanius prohlematicus, Traquair. 

 Plate V. figs. 5-11. 



1898. Lasanius problematicus, Traq., in Director-General's Summary of Progress for 1897, p. 73. 



Specific Characters. — Eighteen median scutes along the ventral line, aculei slender. 



Description. — In ordinary specimens only two things are to be seen — the long row 

 of median scutes and the arrangement of parallel rods in front — and though these 

 always maintain the same relative position, it was impossible so long as no other parts 

 were observed to decide as to which was the dorsal and which the ventral aspect of the 

 fish. That the line of scutes is, however, ventral is proved by the occurrence of a 

 specimen of the closely-allied species L. armatus, in which the arrangement of the rays 

 of the heterocercal tail-fin can be clearly detected. 



PI. V. fig. 5 represents a typical specimen showing the position of the oblique 

 rods lying above, and passing beyond the anterior extremity of the row of ventral 

 scutes ; the same relations are also shown in the restored outline, fig. 4, in the text. 



Fig. 4. — Lasanius problematicus, Traq., restored outline, enlarged. 

 v.s., ventral scutes ; r. , post-cephalic rods ; r'., chain of ossicles. 



These rods (see also figs. 7 and 8) are eight in number, and consist each of two parts 

 or limbs, which meet above at an angle which increases in acuteness as we pass back- 

 wards along the series, and, where the two parts join, there is a sharp posteriorly 

 directed process. The lower limb, slender and tapering, is directed obliquely down- 

 wards and forwards on the side of the fish, — the upper one is on the other hand short 

 and directed inwards (fig. 7) to meet its fellow of the opposite side in the middle line 

 of the back. In front of the foremost rod there runs parallel with it a row of five or 

 six small ossicles, the lower extremity of each of which rides over the upper extremity 

 of the one below, and each of them (fig. 9) is also furnished with a small backwardly 

 directed thorn-like projection. 



