PLAGIOSTOMI. 1 1 9 



Glass L— PISCES. 



Order 1. — Plagiostomi.* 



(Sharks, Rays). 



Char. — Ed do-skeleton cartilaginous or partially ossified ; exo- 

 skeleton placoidf; gills fixed with five or more gill- 

 apertures; no swim-bladder; scapular arcli detached 

 from the head ; ventrals abdominal ; intestine with 

 spiral valve. 



The earliest good evidence which has been obtained of a 

 vertebrate animal in the earth's crust is a spine, of the nature of 

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

 that of a placo-ganoid fish (p. 138). 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;J its determination to Agassiz, who assigns it to 

 a genus of plagiostomous cartilaginous fishes called Onch/us. 

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

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

 Pteraspis, Knerr. 



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 Siluridce), 

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

 grooved lengthwise, with rounded ribs between the grooves. 

 They are referred to two species — Onchus Hurchisoni and 0. 

 semistriatus. Sir P. Egerton has lately figured another species 

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



* Gr. JPlagios, transverse ; stoma, mouth, in reference to the shape of the 

 mouth, like a transverse slit on the under side of the head. 



f Gr. Plax, a plate ; eidos, form ; the scales being represented by bony 

 tubercles or plates. 



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



