/. — Pleuracanthidce. 709 



pointed out that the genera Dittodus, Aganodus, Pternodus, and Ochlodiis, described 

 by Prof. Owen, are all referable to Diplodus, and had been established on varying 

 examples, more or less fragmentary, of the teeth of that genus.* 



A Paper was published by the writer in 1880,§ on the genus Pleur acanthus, 

 Agass., in which several new forms of spines are described, principally derived 

 from the cannel coal at Tingley, near Leeds, but also from the Staffordshire coal 

 field and Lower Coal Measures of the West Riding of Yorkshire. These spines 

 exhibited a number of intermediate forms between Pleur acanthus Icevissimus, Ag., 

 and Orthacanthus cylindricus, Ag. The latter possesses two rows of denticles, 

 which are situated comparatively close together on the median posterior surface, 

 whilst the former has also two rows of denticles on the sides of the spine and as 

 widely separated as it is possible that they could be. The examples described in 

 the Paper " prove that the difference in the relative position of the two rows of 

 denticles must either be of small generic importance or that many new genera will 

 have to be formed for their accommodation. Almost every intermediate form 

 between the two is now known ; the denticles extend at every angle between the 

 sides and back of the spine." After careful consideration the opinion is enunciated 

 that the several spines were borne by fishes having characters of a single generic 

 type, and that they should consequently be included, under different specific deter- 

 minations, in the genus Pleur acanthus. The Diplodus teeth have hitherto been 

 found indeterminately associated with the spines of Pleuracanthus , Xenacanthtis , 

 and Orthacanthus, and afford additional evidence of their generic identity. 



Professor E. D. CopeJ has recorded the occurrence of more or less complete 

 crania from the Permian beds of Texas. The specimens also include jaws and 

 numerous teeth. The teeth are indistinguishable from Diplodus gibbosiis, Ag., and 

 Diplodus compressus, Newb. The latter is provisionally referred to a distinct 

 genus, and styled Didymodus. Twelve more or less well-preserved crania were 

 examined, one of which exhibited the jaws with teeth and a part of the cranium. 

 The skull formed a continuous piece with distinct segmentation ; it was elongated 

 posteriorly and abbreviated anteriorly, the orbit occupying a position on the 

 anterior third of the cranium with well-defined pre-orbital and post-orbital 

 processes. The top of the muzzle is described as being " excavated by a 

 fontanelle which does not extend posterior to a line connecting the pre-orbital 

 processes." The occipital elements form a wedge-shaped body divided medially 

 by a suture with the apex forward. A second triangular bone is the parietal; its 

 apex is concealed beneath the free extremity ■ of the bone preceding it. On each 

 side, between the occipital and parietal elements are bones which Professor Cope 



* Trans. Odont. Soc, vol. v., 1867. 



f Quart. Journ. Geol. Soc, vol. xxxvi., p. 321, pi. xn. 



J Proc. American Phil. Soc, vol. xxi., p. 572, with plate. 1884. 



