H. Bolton — On the Genus Listracanthus. 425 



Description of Specimens. 



1. Listracanthus spinatus, sp.n. 



Diagnosis. — Spine small, not exceeding 19 ram. in length, gently- 

 arched, sides flat, with seven longitudinal ridges, the convex and 

 concave margins bearing acute spine-like denticles. The denticles 

 of the convex margin few in number and placed at equal distances 

 apart, these pointing a little forwards and upwards, and the angle 

 which they make with the margin being not more than ten degrees. 

 Only three denticles shown, but a fourth probably given off just 

 above the broken upper edge. The first lateral ridge directly pro- 

 longed into the lowest denticle of the convex margin, the second 

 ridge giving origin to the second and third denticles, and probably 

 continuing upwards to a fourth (though a slight dislocation of the 

 spine at this point prevents a clear determination). The denticles of 

 the concave margin larger, stronger, and more numerous than those 

 of the convex margin, and directed outwards at an angle fully double 

 that of the latter. A cluster of five spines placed at the base, in 

 fan-fashion, the margin spreading out between them like a web. 



Listracanthus spinatus, sp.n. Lower Coal-measures, Lancashire. 



Above the cluster are nine others at increasing distances apart. 

 The lateral longitudinal ribs seven in number at the base, but 

 only four continuing up to the broken apex, the remaining three 

 running out into the denticles; all showing traces of slight nodular 

 enlargements. 



Observations. — The species thus described differs in several 

 respects from all the known species, and is well characterized by 

 the differences of the denticles of the concave and convex margins, 

 the slight nod ulation of the lateral longitudinal ridges, and the 

 fan-like expansion of spines at the base of the concave margin. 



Horizon. — Roof of Bullion Mine, Lower Coal-measures. 



Locality. — Lancashire. 



Type in the Royal Museum, Peel Park, Salford. 



2. Listracanthus, sp. indet. 



Description. — Spine small, not exceeding 11 mm. in length, 

 arched, sides flat, with nine longitudinal ribs. Concave margin 

 only bearing denticles, which are small, about twelve in number, 

 and closely set. Convex margin entire. 



Observa lions. —This specimen differs in several important features 

 from Listracanthus spinatus. It is shorter, more arched, and broader 

 in comparison to the length, whilst only the concave margin bears 

 denticles. So little is known of the nature and position of the 

 two spines, that it may possibly be that the differences between 

 them are only due to the position they occupied upon the body, 

 and not to specific difference. Against this view must be placed 



