NORTH STAFFORDSHIRE COAL-FIELD ORGANIC REMAINS. 47 



Silverdale, have yielded fine examples of this fish — in the latter case a frag- 

 ment of a jaw 16 inches in length. With this fish are frequently associated 

 in the New and Knowles ironstones large jaws referred to Rhizodus, and 

 others as yet not satisfactorily determined. 



The small cycloidal-scaled fish assigned to Rhizodopsis is also of common 

 occurrence, and with it are occasionallyfound large circular scales,with concen- 

 tric and radiating lines of ornamentation. Acanthodes is also of wide range, 

 heing more abundant in nodules of the Cockshead ironstone, and in the 

 Knowles and Deep Mine ironstone shales, which also contain Gyrolepis and 

 Pygopterus. Strepsodus, a long somewhat curved tooth with parallel lines, 

 also ranges from the Stinking Coal to the upper beds, but always in detached 

 forms. Amblypterus so far appears to be extremely rare in these fields. 



The Placoid order of fishes is freely represented, and possesses an equally 

 determinate range with the ganoid forms of the field. Pleuracanthus, or the old 

 Diplodus gibbosus, occurs in thousands in the New Ironstone at Fenton Park, 

 making its first appearance in the shales of the Stinking Coal, and ranging 

 upwards to the Red Shag ironstone of the upper beds. The spine described 

 as Pleuracanthus, is frequently found associated with the teeth and dermal 

 covering of the fish, in a manner which confirms the now received opinion 

 of their common origin. Pleura canthus minutus is not so common, but it 

 has a wide and general range, and another species is not unfrequent in the 

 New Ironstone of Fenton Park, but it is rarely found to pass from this 

 horizon. Of the large dorsal spines, Ctenacanthus, the species hybodoides is 

 the most familiar, but it is seldom found out of the New Ironstone, Deep 

 Mine, Knowles and Brown Mine shales. Gyracanthus formosus is of far 

 wider distribution, being found in twelve different beds of the lower and 

 upper thick measures. G. tuberculatum is frequently associated with it, but 

 it is of less vertical range. Orthaeanthus cylindricus is also confined to the 

 upper part of the thick coal and the lower part of the ironstone groups, but, 

 like Ctenacanthus and Gyracanthus, the finest specimens are obtained from 

 the New Ironstone of Fenton Park. The fossil known as Onchus is remark- 

 ably persistent in its distribution, being plentiful in the Stinking and Wood- 

 head coal-shales, and in nearly the whole of the coal and ironstone beds of 

 the succeeding measures. A form resembling Leptacanthus, and others with 

 double or single rows of curious hooked teeth, are not unfrequently met 

 with, especially in the New Ironstone, which appears to have been the great 

 life-centre of the singular animals to whom those defensive weapons 

 belonged. 



The remainder of the Placoid fish consist of the genera Cladodus, Peta- 

 lodus, Gtenodus, Orodus, Ctenopty chins, Archodus, Pcecilodus, Helodus, some 

 forms resembling Cochliodus, a small tooth with numerous branching cusps, 

 and others as yet undetermined. Gtenodus appears to be limited to the New, 

 Brown Mine, Knowles, and Black-band ironstones, and is of somewhat rare 

 occurrence. Petalodus is of similar range, and Cladodus extends to the 

 Woodhead coals, and is frequently found in groups on slabs of the Knowles 

 ironstone and Ash-coal shales. Orodus is confined to the Stinking Coal of the 

 lowest measures, and also another form of an exceedingly interesting character, 

 of which but two specimens have been detected. Ctenoptychius is represented 

 by three or four species, of which ap'ualis is the most common, the New 

 Ironstone containing remarkably fine examples. Archodus is a rare form, 

 and appears to be confined to the latter bed. To Helodus no limit can well 

 be given, it being met with in almost all the beds, and in the Knowles is 

 frequently of large size. The tooth resembling Cochliodus is rare, and has 



