104 PALEONTOLOGY 



Pleuracanthus, therefore, as Agassiz concludes, may offer the 

 earliest example of the flat form of cartilaginous fish which is 

 represented by the sting-rays (Trigon, Mylidbates) in the 

 present seas. The ichthyodorulite (ichthys, a 

 fish ; dora, a spear ; lithos, a stone) here selected 

 to illustrate this fossil, is a portion of the' spine 

 of the Pleuracanthus levissimus (fig. 25), from the 

 carboniferous beds near Dudley. The other 

 plagiostomous genera based upon fossil spines 

 from the coal formations are, — Oracanthus, Gyra- 

 Fig. r 25. canthus, Nemacanthus, Cosmacanthus, Leptacan- 

 PUuracanthus thus, Homacanthus, Trustichius, Astcropteri/chius, 



levissimus. 



(Coal, Dudley.) Physonemus, Sphenacanthus, Platy acanthus, Dip- 

 riacanthus, Erismacanthus, Orthacanthus, Cladacanthus, 

 Lepracanthus. 



Immediately above the coal measures lie a variable series 

 of sands and clays of different colours, including the coal 

 plants ; above this, a marl slate in thin layers, containing 

 scanty evidences of fishes ; but these are more abundant and 

 instructive in the superincumbent magnesian limestone, in 

 which formation, near Belfast, ichthyodorulites of the genus 

 Gyropristis (Ag.) have been found. Above this are the penean 

 red sandstones, in which, at Westoe, have been found fossil 

 spines closely allied to, if not identical with, the Gyracanthus 

 formosus (Ag.) The foregoing formations constitute the upper- 

 most of the palaeozoic series called " Permian," from the 

 Russian province in which these strata are most extensively 

 developed. Their relative position is known by the term 

 " magnesian limestone " in the " Table of Strata," fig. 1. 



The superimposed strata, marked " new red sandstone," 

 includes also a varied series of red and white sands, marls, 

 and conglomerates, forming collectively the system called 

 " triassic." The ichthyodorulites of this system are referable 

 to the genera Nemacanthus, Leiacanthus, and Hybodus. In 



