J. W. DATIS OX THE GEXTJ8 PLEUR ACANTHUS, AGASS. 335 



surfaces are round and smooth ; the posterior is depressed, one tenth 

 of an inch across at the base, contracting to a small groove towards 

 the apex ; the angles formed by the contact of the posterior and 



Fig. 10. — Pleuracantlius denticidatus ^ Davis. 



Denticles of the spine, enlarged. 



lateral faces are armed with a series of closely set denticles (fig. 10), 

 which are small, comparatively broad at base, contracting suddenly, 

 then forming a carinated apex, pointed at the extremity towards 

 the base of the spine. There are 20 denticles on each side in the 

 space of an inch ; and in the specimen figured there are 45 on each 

 side. A circular cavity (which appears, as usual, to be terminal) ex- 

 tends towards the point ; it is large in diameter in proportion to the 

 spine. 



Locality. Better-bed Coal, Clifton, near Halifax. 



Several specimens which have been obtained from the Cannel 

 Coal at Tingley may probably be referred to this genus. They pre- 

 sent some points of difference ; but these may probably be accounted 

 for by the relative position of the two stratigraphically, the Cannel 

 Coal being several hundred feet higher in the Coal-measure series 

 than the Bone-bed. The Tingley spines are slightly compressed 

 laterally ; the anterior and lateral faces are striated near the basal 

 extremity ; the posterior denticles are small and much resemble 

 those of the Bone-bed specimens ; the base is better exposed, its walls 

 are rather thin, and the internal cavity is large ; the spines ex- 

 pand towards the base. A perfect example will be about 3*5 inches 

 in length. 



P. denticidatus appears to be most nearly associated with P. 

 Wardi, and with a spine described by Dr. Newberry (in the Palae- 

 ontological volume of the ' Survey of Ohio,' vol. i. p. 332, pi. lix. 

 fig. 4) as Ortliacanilius arcuatus. The latter is described as finely 

 striated longitudinally on the anterior surface — the posterior surface 

 occupying one third the circumference, and having a low ridge along 

 the median line. The denticles extending along the latero-posterior 

 angles are much closer together and more numerous, and from the 

 figure appear to be quite different in character from those in my spe- 

 cimen. In all these particulars the two species arc clearly divergent ; 

 in other respects they are similar ; in curvature and general form 

 they are evidently very closely related. 



P. denticidatus is easily distinguished from P. Wardi by the largo 

 size of the latter, the pecuHar squareness of its lateral faces, and 

 its obtuse teeth connected together by intermediate ridges. In 

 these respects they differ essentially from each other. 



