HARD WICKE ' S S CIE NCE - G SSIP. 



much as two feet in length. M. paradoxus and 

 M. retrorsus are the larger species, and M. granulosus 

 is one of a more diminutive kind. All occur at Lyme 

 Regis. 



But the most prominent of the Mesozoic ichthyo- 

 dorulites is a form particularly remarkable for its 

 ornamentation by star-shaped bosses of enamel, and 

 hence termed Astcracanthus ; so striking, indeed, is 



from Swanage, and of A. granulosus from Tilgate 

 Forest. The ornamentation is much finer in these 

 Wealden and Purbeck species than in that of the 

 Kimmeridge Clay, and the tubercles more closely 

 crowded together, and the genus seems to have 

 completely disappeared before the commencement of 

 Cretaceous times. 



It is curious that hitherto there have been no 



Fig. i. — Erisiuacauthits Joncsii (after J. W. Davis). 



If 



w- 



mi 



*- 



If 



i 



:<*%\ 



w 



* 



A* 



w 



:■ 



the appearance of this spine, 

 that it was one of the earliest 

 studied of fossil bodies, being 

 described and figured in the 

 Philosophical Transactions of 

 the Royal Society in 1753, and 

 quaintly referred to " the head 

 or snout of some animal of the 

 fish kind, or perhaps of some 

 lizard, alligator, or crocodile." 

 The genus is first represented by 

 a few doubtful fossils in the Lias, 

 but Agassiz has definitely de- 

 termined species from several of 

 the succeeding Jurassic beds, 

 and Sir Philip Egerton* has also 

 made known the presence of 

 others in the Purbeck and Weal- 

 den. The largest and typical 

 species — sometimes more than 

 twenty inches long— is A. orna- 

 tissimus (fig. 2), from the Kim- 

 meridge and Oxford Clays, and 

 exhibits well the double row of 

 posterior denticles, the extensive 

 inner cavity, and the long base 

 of insertion. Among the Lower 

 Oolite forms are A. semisulcatus, 

 occurring in the Stonefield Slate, 

 near Oxford, and A. Stitchburii, in the Forest Marble 

 of Dorsetshire ; while the evidence of the genus in 

 the Purbeck and Wealden consists in beautifully 

 perfect spines of A. verrucosus and A. semiverrucosus 



Fig. 2. — Astcracau- 

 thus ornatissimus 

 (half nat. size, after 

 Agassiz). 



* Mem. Geol. Surv., Dec. VIII., pi. i.-iii. 



recorded instances of the discovery of Asteracanthus 

 in intimate association with teeth or other Selachian 

 structures, and nearly fifty years have thus passed 

 without our advancing beyond the mere surmise 

 of Agassiz, who thought that Stropkodus might 

 eventually prove to be the dentition of the same 

 cartilaginous form ; Sir Philip Egerton, however, in 

 describing the Purbeck spines, has pointed out that 

 such an idea can scarcely be probable now, since 

 Stropkodus is quite unknown in the well-explored 

 strata of Swanage, and more complete evidence must 

 yet be awaited before there are grounds for removing 

 the genus from its present provisional place. 



We have now reached the end of the task proposed 

 in our programme of August, 1884,* and would 

 venture finally to express the hope that this cursory 

 glance at the present state of knowledge in regard to 

 the past history of Sharks and Rays may not be 

 without the result of contributing, however slightly, 

 towards its advancement. It has been our endeavour, 

 as far as possible, to indicate where present infor- 

 mation is most defective, and where those who are 

 the fortunate possessors of fine specimens and yet 

 have not access to the more abstruse literature of the 

 subject, will be furthering the cause of biological 

 science by making them known ; and the writer will 

 deem it a favour and a pleasure to be informed of the 

 existence of such new materials, and to furnish more 

 precise particulars concerning any of the facts or 

 inferences briefly touched upon above. 



Appendix.- It has not been thought necessary to give de- 

 tailed references to the great work of the founder ot Fossil 



* The articles of this series have appeared as follows : — 

 1884— August, October, December; 1885— May, July, October, 

 December. 



