DR RAMSAY H. TRAQUAIR ON THELODUS PAGEI. 599 



Fig. 1 on the accompanying Plate represents the specimen in question, one-fourth 

 smaller than the natural size. The entire length is 12 inches, the breadth at the 

 widest part, namely, between the two lateral fin-like expansions, is 6 inches. 



The fish, as it lies before us, may be described as consisting of two divisions, anterior 

 and posterior. The former of these, broad and expanded, may be said to be approxi- 

 mately trapezoidal in shape, the narrow side, which is also convex anteriorly, being 

 in front, the broad one behind, while the opposite lateral margins diverge as they pass 

 backwards ; and we note further that the angles are rounded off. Or, in other words, 

 the anterior vertically compressed and laterally expanded portion forms at each posterior 

 rounded angle a fin-like flap, the contour of which is sharply marked off behind from 

 the tail, but is continuous anteriorly with the outline of the head, which is also rounded 

 in front. 



These flap-like expansions were by Powrie considered to be pectoral fins comparable 

 with those of the Rays, and for my own part I cannot see how we can interpret them 

 otherwise than as lateral fin-folds. If that interpretation is correct, we have here a 

 very interesting point in connection with the much discussed question of the mor- 

 phology of the paired limbs in vertebrates. 



The surface contained within the boundaries of this expanded anterior part of the 

 fish is gently convex. About the middle, however, there is a slight longitudinal depres- 

 sion about 2| inches in length, and reaching to 1^ inch from the front, at the bottom 

 of which, and extending for its whole length, there is a feeble bar-like elevation or 

 rounded ridge. From this central longitudinal ridge there pass out on each side eight 

 transverse bar-like elevations of a similar character, the more posterior of which tend 

 more and more to a backward direction, so that the last pair show a contour like that 

 of a V widely open and reversed in position. 



I have not the slightest doubt that Powrie was right in interpreting this arrange- 

 ment of ridges and intervening grooves as indications of a branchial apparatus, but it is 

 quite another thing to describe the " branchiae " as " consisting of exposed arches," or to 

 deduce from the presence of those markings that we have the ventral aspect of the fish 

 before us. We see neither branchiae nor branchial openings in the specimen, and the 

 ridges and grooves alluded to are, to my mind, simply indications of the presence of 

 a cartilaginous branchial skeleton in the living fish, but which of course is lost in the 

 fossil. That the exposed aspect of the specimen is dorsal and not ventral, is, as we shall 

 see further on, conclusively shown by the position of the lower lobe of the heterocercal 

 caudal fin. 



It seems, therefore, certain that this expanded anterior part contains much more 

 than the head, — namely, the anterior portion of the body as well, — and in this manner the 

 lappet-like expansions which form its right and left posterior angles may well be inter- 

 preted as lateral fin-folds. Their general appearance also suggests an analogy with the 

 cornual flaps of Cephalaspis, which were originally considered by Lankester as the 

 equivalents of pectoral fins. 



