] 00 PALAEONTOLOGY 



of the dorsal spine of the dog-fish (Acanthias), and a buckler 

 like that of Cephalaspis. Both have been found in the most 

 recent deposits of the Silurian period, in the formation called 

 " upper Ludlow rock." The discovery of the first is due to 

 Murchison ;* its determination to Agassiz, who assigns it to 

 a genus of plagiostomous cartilaginous fishes called Onchus. 

 The buckler was discovered by Mr. Banks, in the " passage- 

 beds " of Kington, Herefordshire, and is referred to the genus 

 Pteraspis, Knerr, with microscopic evidence of its piscine 

 nature, by Huxley. 



The Onchus spines from the upper Ludlow bone-beds are 

 compressed, slightly curved, less than two inches in length, with 

 no trace at their base of the joint characteristic of the dorsal 

 spines of the " sheat-fishes " (Ganoids of the family Sihiridce), 

 or "file-fishes" (Balisticlce). The sides of the spine are finely 

 grooved lengthwise, with rounded ribs between the grooves. 

 They are referred to two species — Onchus Murchisoni, and 0. 

 semistriatus. Sir P. Egerton has lately figured another species 

 from the argillaceous beds near Ludlow, which is more curved, 

 and is armed along the posterior edge ; the longitudinal ribs 

 are fine and numerous, but are constricted at intervals, as in 

 the genus Ctenacanthus, and become subtuberculate at the 

 base. He deems them significant of a distinct genus of shark- 

 like fishes.f 



With the dorsal spines of Onchus are found petrified por- 

 tions of skin, tubercular and prickly, like the shagreen of 

 shark's skin, and referred to a genus called Sphagodns ; also 

 coprolitic bodies of phosphate and carbonate of lime, including 

 recognisable parts of the small Mollusks and Crinoids which 



* Silurian System, ch. xlv., p. 606. 



f In a formation in Indiana, United States of America, referred by Messrs. 

 Norwood ami Dale Owen to the Silurian formation, a badly-preserved fossil, 

 considered as an IcLtliyolite, and referred to a genus allied to Plericlrfhys, has 

 been discovered, and called Mucropetalichthiis ruphi'ulolabis. (Silliman's 

 Journal, 1846, p. 367.) 



