I. — PleuracantJiidce. 717 



than the majority, which probably occupied a posterior position. The lateral 

 denticles are comparatively strong, and they are considerably bent inwards ; the 

 median denticle is shorter and thicker than that of the teeth further back 

 (PI. Lxvi., fig. 2). From an examination of the examples figured it will be observed 

 that they offer a considerable variation in form, and taken independently would 

 probably have been considered to represent fishes of distinct species. 



The shoulder-girdle, together with bony masses of the branchial arches, are pre- 

 served on a slab obtained from the roof of the Lowmain coal seam near Newcastle, 

 now in the collection of W. Dinning, Esq., of that city. The bones of the scapular 

 arch to which the pectoral fins were attached (PL lxvil, fig. 1 a, b) occupy approxi- 

 mately a natural position. They are large, well-developed chondroid structures. 

 The scapular elements enumerated by Dr. Anton Fritsch* appear in this sjjecimen to 

 be fused into one piece, no lines of demarcation being visible. The central portion 

 of the specimen is occupied by portions of the branchial arches, those on tbe left 

 side (c, Ci, Cj, Cs, c^) being five in number, whilst on the right only four can be dis- 

 tinguished {d, di, d^, di). A large and massive osseous element (e), partly projecting 

 and partly under the right scapular arch, is probably the hyo-mandibular. Scattered 

 over the slab are a number of the small stemmatoid ossicles (fig. 1^) previously 

 described, which have been separated from the branchial arches. A peculiar bone 

 of a tripartite character occupies a position behind those mentioned above. It has 

 a length of 0'025 m. along its median axis, extending antero-posteriorly (fig. 1/), 

 at a distance of about one-fourth its length from the posterior extremity ; a branch 

 extends from each side, O'Oll m. in length; O'OOS in diameter at the base of 

 attachment, and diminishing to a point at the distal extremity. The derivation of 

 this bone is not quite clear, but it may have been attached to the occipital region 

 of the skull, and served as a base for the attachment of the cephalic spine. 



Dr. Anton Fritsch describes the genex-a Fleur acanthus and Xenacanthus as 

 being possessed of seven gill arches.f The principal evidence is afforded by a 

 specimen of Xenacanthus Decheni, Groldf., from the limestone of Oelberg, near 

 Braunau. The specimens of Pleuracanthus are from the gas coal of Tremosa, near 

 Pilsen, and are more or less imperfect. Dr. Fritsch intimates that the true defi- 

 nition of the gill-arches is very difficult, and so far as the Pleuracanthus is con- 

 cerned he should not hold the evidence sufficient were it not for the proof that 

 the allied genus Xenacanthus could be shown to have had seven gill-arches on each 

 side. The first arch is weak ; the second to the fourth are similarly strong and 

 longer ; the fifth is shorter ; the sixth still shorter ; and the seventh is much thicker 



* Fauna der Gaskohle, vol. iii., pt. i., p. 41, fig. 240 (woodcut). 



f Fauna der Gaskohle, vol. iii., pt. i., p. 8, pi. xcm., fig. 3 ; pi. xciv., fig. 1 ; and p. 25, pi. xcvi., 

 fig. 1, woodcuts 193- and. 215. 



