380 Correspondence. 



of that fish. Thus it would appear that Mitrodus, instead of be- 

 longing to a fish of the size of a minnow, is most likely part of 

 one of the largest, if not the very largest, fish of the Coal-measures. 

 Bittodus too — as the size of the dermal patches proves — was an 

 animal of no mean dimensions. 



Genus III. Ageleodus. — This is the Gtenoptychius of Agassiz. It 

 occurs of various sizes, and is sometimes minute; but it is fre- 

 quently upwards of three-tenths of an inch wide, and is usually 

 found detached, though sometimes two are placed end to end ; tliis 

 form has also much the appearance of being a dermal tubercle 

 rather than a tooth. It is much compressed, and the spines, or 

 "dental crowns," which vary in number from six to sixteen, give to 

 the upper margin a saw-like denticulation. The bony base, giving 

 support to the denticulated portion is, of considerable extent, and has 

 all the appearance of the basal limb of a dermal tubercle. 



Genus IV. Ganacrodus. — Teeth tipped with enamel are very com- 

 mon in the shale of the Low Main Seam ; they vary greatly in size, 

 though not in other respects, but when seen in section they seem to 

 differ considerably. Their apparent curvature depends entirely on 

 the plane of the section, and the point becomes more or less obtuse 

 as this is further from, or nearer to, the axis of the tooth. The 

 apparent extent and form of the enamel-tip also varies much in 

 accordance with the section. Some specimens are entirely coated 

 with a thin film of enamel ; in others, traces of it only can be ob- 

 served ; and in some again, it seems entirely wanting, owing 

 apparently to the state of preservation of the sj)ecimen. 



I have little hesitation in saying that all these teeth tipped with 

 enamel found in the shale overlying the coal of the Low Main 

 Seam at the localities before-mentioned, belong to PalcEoniscus — the 

 teeth of which genus are always thus tipped with enamel, as I have 

 satisfied myself by sections of several jaws, with the teeth attached, 

 taken from well authenticated specimens. 



That Palaoniscus occurs abundantly in this shale I have ample 

 proofs, having taken from it well preserved individuals of this 

 genus, besides numerous fragmentary specimens. The teeth of 

 Amblypterus and Pygopterus are likewise tipped with enamel, but the 

 latter genus has not yet been found in the localities named. 

 Amblypterus is very rare : the tooth of Pygopterus is extremely 

 characteristic, and is readily distinguished by the obtaseness of the 

 terminal enamel. 



The teeth of Palceoniscus vary much in size in the same individual, 

 some being quite microscopic, others comparatively very large. 

 The minute ones are exceedingly numerous, and much crowded ; the 

 larger are few in number, and considerably apart from each other. 



Genus V. Ganalodus . —There are two of this number. Genus 5, 

 Mioganodus, and Genus 6, Aganodus, are probably all founded on 

 teeth belonging to either Palceoniscus or Amblypterus, both of which 

 genera occur in the shale connected with the Low Main. 



Genus VII. Pternodus. — When a single spine of Diplodus is ob- 

 served in lateral section, it has a heeled appearance, and in other 



