4 Dr. H. Woodward — Note on an Ichthyodorulite — 



prolongations of the enamelled base of the superior denticle, which 

 reaches back to its middle point. The spine is segmented throughout, 

 each segment bearing a denticle ; the segments overlapping to such 

 a degree that the one bearing the superior denticle reaches two- 

 thirds of the distance from the summit to the base of the spine." 



A comparison of the Australian fossil spine with the American 

 species of Edestus (excellent casts of both of which are in the 

 British Museum, Natural History) has convinced me that its true 

 place is in or near to Leidy's genus. The denticles are placed 

 on the convex edge of the spine, a peculiar feature in the genus 

 Edestus, nearly all the spines of fossil plagiostomous fishes being 

 denticulated along the inner concave curved edge. 1 The imbricated 

 curved lines passing from each denticle obliquely across the shaft of 

 the spine are also seen in both, and are quite peculiar to Edestus. 



Prof. Newberry has clearly pointed out the objections to the theory, 

 at first suggested by Prof. Leidy, that Edestus was part of the 

 mandible of a plagiostomous fish. He mentions that " the jaws of 

 sharks are always cartilaginous, holding the bony and enamelled 

 teeth only by ligamentous attachments, so that in a fossil state the 

 jaws have usually disappeared, and the teeth are scattered, whereas 

 in Edestus we have a dense bone to which the tooth-like denticles 

 are united by a firm bony anchylosis." Again, " The form of the 

 fossil is wholly unlike that of any jaw of fish, reptile, or mammal 

 known, being rounded below, and terminating in an acute point, 

 smooth, even polished, and evidently never having been covered 

 by integument." 



In comparing the Australian fossil with various specimens in the 

 collection, Mr. William Davies drew my attention to the similarity 

 in arrangement of the teeth of Janassa bituminosa, from the Permian 

 of Durham ; in one specimen of which six teeth with prolonged and 

 highly-curved bases may be seen united ; but these teeth differ in 

 arrangement from the denticles of Edestus in being placed broad- 

 wise behind each other, not edge-to-edge in a row, as in Edestus. 



The only structure amongst fossil fishes strikingly analogous to 

 Edestus is to be found in the pectoral fins of the genus named by 

 Agassiz Ptychodus, from the Chalk of Sussex, and since described 

 by Prof. E. D. Cope from the Chalk of Kansas, under the name of 

 Pelecopterus. 



Professor Cope writes : — " The entire pectoral fin, so far as is 

 known, is devoted to the construction of a powerful spine. This 

 follows from the fact that the spine is supported by all the 

 basilar bones. Six of the latter articulate in the fossae of the groove 

 of the scapula. They are flat, contracted at the middle, and ex- 

 panded at the extremities. In front of these are two others of a 

 short, thick, cylindric form, one applied to the superior, the other 

 to the inferior facets of the scapula above mentioned, while the 

 tuberosity rises pedestal-like between them. This structure gives a 

 slight hinge-movement like the opening of the blade of a knife, and 



1 In Phuraeanthus the spines are barbed on both edges, and so also are the spines 

 of the Sting-rays. 



