STRUCTURE AND AFFINITIES OF THE PLATYSOMIDA., 365 
mond, Yorkshire.* Chetrodus is, however, chiefly known as a fish of the Coal 
Measures, in which horizon C. granulosus is abundant in North Staffordshire, 
occurring also in a more fragmentary condition both in Lancashire and in 
Lanarkshire, while the smaller C. striatus has been yielded by the coal strata 
of Northumberland. 
Structure.—The body of Cheirodus (Plate V. fig. 1) is deep and rhombic, the 
dorsal and ventral margins elevated into peaks, of which the dorsal is slightly 
in advance of the ventral; the tail is completely heterocercal, but not very 
inequilobate; no ventral fins have been as yet observed, and the pectorals are 
rarely seen, owing to their having been placed a little higher up than usual on 
the side of the flattened body, as indicated by the position of their roots, which 
are always observable behind the lower extremity of the clavicle. The dorsal 
and anal fins arise immediately behind the respective dorsal and ventral peaks 
of the body, and extend to the narrow tail pedicle; they are therefore nearly 
equal, with elongated bases, and fringe-like for the greater part of their extent. 
Their anterior margins are fulcrated ; their rays are numerous, closely set, and 
closely articulated ; the joints being externally ganoid and sculptured. The scales 
of the body are arranged in bands, which are nearly vertical, with a slight back- 
ward obliquity; just below the insertion of the pectoral fin they curve slightly 
forwards, as in Mesolepis and Benedenius, turning also slightly backwards just 
in front of the dorsal and ventral peaks. They are very high and narrow, 
especially on the flanks, above and below they become lower ; while on the 
caudal body prolongation they are very small, lozenge-shaped, and arranged in 
oblique rows, whose direction is from above downwards and forwards. The 
scales are also peculiarly modified on the anterior margins of the dorsal and 
ventral peaks, where they assume a spur-like appearance, with backwardly 
directed points; on the upper margin of the caudal body prolongation they have 
the usual V-shaped aspect and imbricated arrangement. In the body scales the 
harrow anterior covered area is very distinctly marked off from the sculptured 
one, which in C. granulosus (Plate V. fig. 13) is ornamented with closely set 
tubercles, which sometimes assume a more or less linear arrangement, while 
in C. striatus they coalesce on the scales of the middle part of the body into 
actual though somewhat irregular vertical striz. The articular spine is very 
Strong, its base often extending back along the whole of the upper margin ; 
the vertical rib or keel of the attached surface (fig. 14) is close to the anterior 
margin, and is very strongly and sharply defined. 
The upper contour of the head continues the downward slope of the back 
in front of the dorsal fin, showing also, as in Hurynotus and Mesolepis, a slight 
convexity above and in front of the orbital region, whence its descent to the 
front of the mouth is more rapid. Most of the cranial roof-bones can be made out 
* Geol. Mag. ser. 2, vol. i. 1874, p. 431. 
