118 FISHES OF THE SILURIAN ROCKS, 



unequivocally belong. It is peculiarly the order of the 

 system.* 



The Ludlow bone-bed contains not only defensive spines, 

 but also teeth, fragments of jaws, and shagreen points. Let 

 us, then, take the defensive spine as the part on which to 

 found our comparison. One of the best marked placoids :f 

 the Upper Ludlow bone-bed is that Onchus Murchisoni to 

 which the distinguished geologist whose name it bears refei*s, 

 in his communication, as so nearly resembling the oldest pla- 

 coid yet known, — that of the Bala Limestone. And the 

 living fishes with which the Onchus Murchisoni must be com- 

 pared, says Agassiz, though " the affinity," he adds, " may 

 be rather distant," are those of the genera " CestracioUy Cen- 

 trina, and Spinax^ I have placed before me a specimen of 

 recent SpinaXy of a species well known to all my readers on 

 the sea-coast, the Spinax Acanthias, or common dog-fish, so 

 little a favourite with our fishermen. It measures exactly 

 two feet three inches in length ; and of the defensive spines 

 of its two dorsals, — those spear-like thorns on the creature's 

 back immediately in advance of the fins, which so frequently 

 wound the fisher's hand, — the anterior and smaller measures 

 from base to point an inch and a half, and the posterior and 

 larger two inches. I have also placed before me a specimen 

 of Cestracion Phillippi (the Port-Jackson shark), a fish now 

 recognised as the truest existing analogue of the Silurian pla- 

 coids. It measures twenty-two three-fourth inches in length, 

 and is furnished, like Spinax^ with two dorsal spines, of which 

 the anterior and larger measures from base to point one one- 

 half inch, and the posterior and smaller one one-fifth inch. 



* It is to be regretted that the author had not the placo-ganoids of 

 the Upper and Lower Ludlow rocks to reason upon, as well as the pla- 

 coids. But the inferences from the data he had, — the defensive spines 

 and shagreen of the Upper Ludlow, — are not affected thereby. What 

 he would have added may be gathered from his remarks elsewhere upon 

 the Cephalaspians. — L. M. 



