The Chimaerids are very poorly represented in the British Chalk, and are known from perhaps just a handful of dental plates belonging to the genus Ischyodus, a form more familiar from the Jurassic.  Ischyodus is characterised by a hardened band along the oral margin of the outer surface (see figures 3A and 4A), and multiple outer tritors on the inner surface.  The dental plates of the morphologically similar Edaphodon are generally much larger.

1).  Ischyodus sp.; unique articulated pair of mandibular plates, sharing characters of both I. thurmanni (large median tritor) and I.(?) incisus (multiple posterior outer tritors) - the right mandibular has fallen away, leaving the impression of the inner surface, revealing the arrangement of the tritors (x1.5, Grey Chalk, Southerham lower pit, near Lewes, East Sussex, Shepherd Collection).  Image by kind permission of Roy Shepherd.

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2).  Inner surfaces of Ischyodus mandibulars; (A) Type specimen of I.(?) incisus - left mandibular displaying a narrow median tritor and multiple posterior outer tritors (x1.5, Chalk, Kent, BMNH (British Museum (Natural History) London) 41683, image modified from plate XLI, figure 5a of Woodward 1902-1912); (B) Right mandibular of I. thurmanni, displaying broad median tritor (x1.5; Grey Chalk, Iselham, near Newmarket, Cambridgeshire, image modified from text figure 56A of Woodward 1902-1912).

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3).  Ischyodus sp. (Grey Chalk, Southerham lower pit, near Lewes, East Sussex, Shepherd Collection); (A) Outer surface of left mandibular - note hardened rim to the oral margin (x1.7); (B) Impression of inner surface of right mandibular (x1.7).  Images by kind permission of Roy Shepherd.

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4).  Ischyodus(?) incisus, outer views of left mandibulars, x1.5; (A) Type specimen showing the hardened oral band which defines the genus (Chalk, Kent, BMNH (British Museum (Natural History) London) 416830; (B) Specimen not displaying the hardened oral band (Grey Chalk, Burham, Kent, BMNH 47942).  Images modified from plate XLI, figures 5 and 6 (respectively) of Woodward 1902-1912.

5).  Hydrolagus colliei (Spotted Ratfish); A modern Chimaerid for illustration (image sourced from Wikipedia).

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6).  Ischyodus(?) incisus, left vomerine plate (x1.5, Grey Chalk, Burham, Kent, BMNH  (British Museum (Natural History) London) 47942); A Outer view; (B) Inner view.  Images modified from plate XLI, figures 7 and 7a (respectively) of Woodward 1902-1912.