/. — PleiiracantMdos. 737 



laterally, short, and terminating obtusely, with an elongated cutting edge parallel 

 to the longitudinal axis. They diminish in size towards the point, and for about 

 0"004 m. the point of the spine is free. The surface of the spine is uniformly 

 covered by minute longitudinal striae. In most of the examples the base of the 

 spine is crushed ; its walls are thin, and the internal cavity comparatively large. 



The specimen described as Pleuracanthus alternidentatus^ Davis,* from Middleton, 

 near Leeds, probably belongs to this species. It is slightly more robust and longer, 

 and the spine in section is rounder ; the principal difference is in the more widely 

 separated position of the denticles and their being placed along the margins in 

 alternate series, instead of being opposite. Since the description was written in 

 1881 other specimens have been found in Yorkshire and also in Staffordshire. The 

 latter are particularly interesting because they appear to indicate a passage between 

 the two forms ; the number of denticles is smaller than those of the type specimen 

 of P. alatus, but greater than those of P. alternidentatiis^ and they are placed some- 

 what irregularly. Taking into consideration the new evidence it appears desirable 

 to regard the two forms as one species, and it is now suggested that Pleuracanthus 

 alternidentatus be included as a synonym of Pleuracanthus alatus, Davis. 



A small specimen from the Deepmine shale at Longton, in Staffordshire, from 

 the collection of Mr. John Ward, is 0-017 m, in length; one half the length is 

 armed with a double row of barbs, numbering seven or eight on each side. The 

 characters of this example appear to indicate its relationship to P. alatus, and it is 

 probably the spine of an immature fish. It is worthy of note, however, that the 

 denticulated surface bears a much larger proportion to the whole length of the 

 spine than in the fully grown specimens, and the distal extremity is not so pointed 

 (PI. Lxxiii., figs. 7-9). 



Examples of spines found in the Lowmain coal of Newsham occur in the 

 Atthey collection at the Newcastle Museum. They have a length of 0-045 m. 

 A double row of denticles extends a distance of 0-017 m., as in the type specimen, 

 each row containing sixteen to eighteen denticles, longer, recurved, and more 

 pointed than in the specimens from Tingley or Staffordshire. These character- 

 istics vary considerably from the original ; but the general resemblance of the 

 form is sufficiently close to justify its inclusion. The basal extremity of the speci- 

 men figured is well exposed (PI. lxxiii., figs. 10, 11). 



Mr. James Thomson has furnished examples collected at Newarthill, Quarter 

 Hamilton, and other localities near Glasgow, which have about ten denticles in each 

 row, and approach very near to the type from Tingley (PI. lxxiii., fig. 13). 



Formation and Locality. — Middle Coal Measures; CannelCoal, Tingley and Middle- 

 ton, near Leeds ; Knowles Ironstone Shale, Fenton ; and Deepmine Ironstone, 



* Quart. Joum. Geol. Soc, vol. xxxvi., p. 328, pi. sn., fig. 3. 



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