Remains of the Pycnodont Fish, Mesturus. 11 



parison with the corresponding fin in a specimen of Mesturus 

 verrucosus in the British Museum (no. 37023). The rajs at 

 the anterior border gradually increase in length. The two 

 foremost rays are practically only elongated ridge-scales, the 

 right and left halves of each being fused together at the 

 apex ; the next two rays are also unjointed, but exhibiting 

 the two halves completely separated ; the fifth ray shows 

 three articulations, two being close together and the third a 

 long distance further up ; the succeeding rays are all closely 

 jointed distally. Between the extremities of the second to 

 the filth rays on the anterior border there are four short and 

 delicate regularly alternating intercalary rays, which may 

 perhaps be termed fulcra. 



Squamation. 



The trunk is very incomplete in all the specimens, but 

 considerable portions of the squamation of the abdominal 

 region are preserved. The scales (PI. III. fig. 1) are all 

 thick and quadrangular, not strengthened within by any 

 well-defined rib, but deeply imbricating, and united above 

 and below both by the peg-and-socket articulation and by a 

 jagged suture. This remarkable suture is most pronounced 

 on the middle of the flank, reduced to a slight waviness 

 towards the ventral border of the fish, while in one specimen 

 (no. 23) it is comparatively feebly marked. The external 

 face of each scale is ornamented with large and rounded 

 closely arranged tubercles ; but these are wanting towards 

 the smooth hinder border, which is not denticulated. The 

 very regular flank-series become less regular towards the 

 ventral margin, and in no. 1 one series terminates in a wedge- 

 shaped scale slightly above and in advance of the pelvic fins. 

 Some of the flank-scales are about twice as deep as broad, 

 while those of the ventral region are mostly about as broad as 

 deep. The azygous ventral ridge-scales are very narrow, 

 ornamented as the others, but having some of the tubercles 

 pointed and delicately striated ; they are destitute of any 

 inwardly directed process, and merely unite with the adjoining 

 scales by a jagged suture. 



II. Systematic Determination. 



The generic determination of the fossils thus described is 

 rendered easy by the nature of the scales. The typical species 

 ot Mesturus, M. verrucosus*, as represented in the British 



* A. Wagner, Abh. k. bay. Akad. Wiss., matb.-pbys. CI. vol. ix. p. 338, 

 pi. iii. fig. 1 (1862). 



