/. — Pleuracanthidce. 721 



osseous centres in a cartilaginous framework. They are larger at the proximal 

 end of the series and diminish in diameter towards the opposite one {cf. i., ii., 

 III., IV.). They may be compared with the specimens figured by Dr. Fritsch of 

 the branchial arches of Orthacanthus Kounoviensis, Fr., a large species comparable 

 with this one, in which the branchial arches are composed of a series of four or 

 five separate semi-osseous parts.* 



Separated from, but near the branchial arches, are a number of small denticu- 

 lated ossicles, which were probably attached to the gill-arches. They consist of a 

 series of small, sharp denticles, attached to a broad base. The arrangement of 

 the denticles varies in nearly every example. Those represented on PI. lxx., 

 figs. 2, 3, are from the slab now described ; others have been found in the 

 Coal Measures of Staffordshire, Lancashire, and Yorkshire. 



The example represented by fig. 4 was found associated with the remains of 

 Pleur acanthus, near Wigan ; it has seven irregularly disposed denticles attached 

 to a more or less triangular base. The denticles are elongated, smooth, and 

 pointed. Mr. Gr. Wild has examples which he has collected from the Middle Coal 

 Measures of Lancashire, one of which, with 16 or 17 prongs, is represented by 

 fig. 5. 



Mr. John Ward has found a considerable number of similar objects in the 

 Ragmine Ironstone Shale, at Fenton in Staffordshire, associated with teeth and 

 other remains of Pkuracanthus (figs. 6—10). They exhibit a great variety in the 

 form and arrangement of the denticles, but notwithstanding the difference in the 

 number of the denticles and the varied manner in which they are attached to 

 the base, there is a general similarity of construction which appears to indicate 

 uniformity of purpose. 



Similar objects were found in the fish -bed of the Upper Burlington Limestone 

 of the Lower Carboniferous Rocks of America. Messrs. St. John and Worthen,t 

 who described the remains, considered them of so anomalous and withal variable 

 character as seemingly to indicate representatives of several distinct though 

 closely-allied generic groups. Subsequently they were regarded as the teeth of 

 a single genus, which was named Stemmatodus, and the so-considered teeth were 

 divided into several species. They were supposed to have occupied the tongue 

 or back part of the mouth of fish similar to Dipterus or Ceratodus. Mr. A. Smith 

 Woodward J regards the specimens described by Messrs. St. John and Woi'then, 

 as well as those from Fenton in Staffordshire, as the dermal tubercles of some of 

 the Elasmobranchs, and this opinion is accepted by Mr. John Ward.§ 



* Op. cit., pt. IV., p. 108, pi. Lxxxiv. 

 f Palaeontology of Illinois, vol. vi., p. 328. 1875. 

 t Catalogue Poss. Fishes, vol. i., p. 248. 1889. 



§ Trans, of North Staffordshire Inst, of Min. and Meoh. Engineers, vol. x., p. 153, pi. ii., fig. 22. 1890. 



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