UPPER AND LOWER. 113 



doubtedly belonging to tlie Cestraciont family of the placoid 

 order." In the American Journal of Science for 1846, Pro- 

 fessor Silliman figured, from a work of the States' Surveyors, 

 the defensive spine of a placoid found in the Onondago Lime- 

 stone of New York, — a rock which occurs near the base of 

 the Upper Silurian system, as developed in the Western 

 world ■* and in the same passage he made reference to a 

 mutilated spine detected in a still lower American deposit, 

 — the Oriskany Sandstone. In the Geological Journal for 

 1847 it was announced by Professor Sedgwick, that he had 

 found " defences o/Jishes" in the Upper Llandeilo Flags, and 

 by Sir Roderick Murchison, that the " defence of an Onchus" 

 had been detected by the geologists of the Government sur- 

 vey, in the Limestone near Bala. Sir Roderick referred in 

 the same number to the remains of a fish found by Profes- 

 sor Phillips in the Wenlock Shale. And such, up to the 

 present time, is the actual amount of the evidence with which 

 we have to deal, and the dates of its piecemeal production. 

 Let us next consider the order of its occurrence in the geolo- 

 gic scale. 



The better marked subdivisions of the Silurian system, 

 as described in the great work specially devoted to it, may 

 be regarded as seven in number. An eighth has since been 

 added, by the transference of the tilestones from the lower 

 part of the Old Red Sandstone group, to the upper part of 

 the Silurian group underneath ; but in order the better to 



* " This is the lowest position [that of the Onondago Limestone] in 

 the State of New York in which any remains have been found higher in 

 the scale of organized beings than Crustacea, with the excepticn of an 

 imperfectly preserved fish-bone discovered by Hall in the Oriskany Sand- 

 stone. That specimen, together with the defensive fish-bone found in 

 this part of the New York system, furnishes evidence of the existence 

 of animals belonging to the class vertebrata during the deposition of the 

 middle part of the protozoic siraXsL." — American Journal of Science and 

 ArU -f&r 1846, p. 63. 



