/. — Pleuracanthidce. 727 



but towards the distal extremity circular in section ; surface smooth. Double 

 row of reflexed, acuminate denticles, one on each lateral margin and extending 

 along two-thirds the length of the spine. An internal cavity extends from the 

 base upwards. Towards the distal end the internal cavity is small, lower it is 

 large, and the walls of the spine are thin ; they are frequently crushed. The base 

 of the spine, when preserved, is contracted at the extremity, and the portion 

 embedded in the integuments was not proportionately large. The large groove 

 stated by M. Agassiz* to extend along the inferior surface of the spine does not 

 always exist in the specimens exam-ined ; tbe appearance may have been due to 

 crushing. The only other record of a similar groove is in the spines of Pleuracanthus 

 parallelus, Fr.,"]" from the gas-coal of Nyran, in Bohemia. 



The base of the spine represented on PI. lxxii., tig. 1, is worthy of note. It 

 is widely and rapidly expanded, which is probably, in part at any rate, due to 

 crushing; but this will not account for the whole of the expansion. 



The spines of this species vary considerably in size ; the largest are about 

 0"3 m. in length ; one specimen from Dalkeith, in the British Museum, has a 

 length of 0*35 m. (PI. lxxii., fig. 2). The specimen from Dudley, figured by 

 Agassiz, is 0'22m. ; % others from the Lower and Middle Coal Measures of the West 

 Riding of Yorkshire attain the same length, whilst the graceful and well-preserved 

 examples from the Staffordshire coal field are mostly about 0'15 m. in length. 



Associated with the larger spines of the cannel coal, in the Middle Coal 

 Measures of the West Riding at Tingley, there are a number of small spines, which 

 have been previously described as Pleuracanthus pulchellus § ; they are generally 

 the same length, about, 0'04 or 0'05 m. Since describing these exquisitely beautiful 

 little spines, when it was suspected that they might be the spines of immature 

 fishes of P. Icevissimus, the discovery of other examples has led to the conviction 

 that such is their proper location, and that the difference in the number of lateral 

 denticles, there being twenty on each side of the small examples against fifty in the 

 large ones, may be due to the respective ages of the two, and increased growth of 

 denticles as the spine has matured. The small imperfect spine named, but not 

 described, by Agassiz, || Pleuracanthus j^^^nus, originally in the collection of Sir 

 Philip Egerton, and now in the British Museum, is about half-an-inch in length, 

 the basal end absent ; the exposed surface is smooth and flat, and six or seven 

 strong denticles extend along each lateral margin. It is recorded as coming from 



* Poiss. foss., vol. iii., p. 66, pi. sxv., fig. 5. 



f Fauna der Gaskohle und der Kalksteine der Pennformatioii Bolimens, Band m., Heft. 1, pis. xci. 

 and xcrv. 



% Poiss. foss., vol. iii., pi. xiv., fig. 6. 



§ Quart. Journ. Geol. Soc, vol. xxsvi., p. 327, pi. sji., fig. 2. 



II Op. cit., p. 177. 



TEANS. EOT. DTJB. SOC, N.S. VOL. IV., PAET XIV. 5 



