The dorsal fin spines of Chimaeras are encountered rarely in the British Chalk.  These are typically attributed to either Edaphodon (slightly curved) or Elasmodectes (straight), based on rare associations of dental plates with spines.  Spines of Ischyodus from other formations are evidently similar to Edaphodon, but presumably would be smaller in the Chalk.

1).  Edaphodon sp. - Large dorsal fin spine, free of original matrix (plaster base).  x0.7, Grey Chalk, Upper Zig Zag Chalk (C. guerangeri zone), between Cow Gap and Holywell, east of Beachy Head, East Sussex, Alan Prowse Collection.

2). Elasmodectes sp. - Dorsal fin spine with portion of pectoral arch.  x3.0, Grey Chalk, West Melbury Marly Chalk, Clayton, near Brighton, West Sussex, Willett Collection, Booth Museum, BMB 007269, by kind permission of John Cooper. 

3). Modern chimaera for comparison, displaying prominent fin spine associated with the dorsal fin.  Modified from image posted by Washington University News.

4).  Edaphodon sp. - Detail of large dorsal fin spine (Figure 1).  Tip of spine showing serrated ornament on inner edge.  x1.6, Grey Chalk, Upper Zig Zag Chalk (C. guerangeri zone), between Cow Gap and Holywell, east of Beachy Head, East Sussex, Alan Prowse Collection.

5).  Edaphodon sp. - Detail of large dorsal fin spine (Figure 1).  Mid-portion of spine showing serrated ornament on inner edge.  x1.7, Grey Chalk, Upper Zig Zag Chalk (C. guerangeri zone), between Cow Gap and Holywell, east of Beachy Head, East Sussex, Alan Prowse Collection.

6).  Edaphodon sp. - Detail of large dorsal fin spine (Figure 1).  Base of spine showing serrated ornament on inner edge.  x1.7, Grey Chalk, Upper Zig Zag Chalk (C. guerangeri zone), between Cow Gap and Holywell, east of Beachy Head, East Sussex, Alan Prowse Collection.