38 DEVONIAN ERA. 



suggest its propinquity to the invertebrate part of creation . 

 The pterichthys has also strong bony plates over its body, 

 arranged much like those of a tortoise, and has a long 

 tail; but its most remarkable feature, and that which has 

 suggested its name, is a pair of narrow wing-like appen- 

 dages attached to the shoulders, which the creature is 

 supposed to have erected for its defence when attacked by an 

 enemy. 



A group of ganoids seem to have been the police of their 

 day, possessing a powerful development of sharp conical 

 teeth situated on the margin of the jaws. One genus, the 

 holoptychius, introduced near the close of the Old Red era, 

 and passing up into the next, presents a flat oval form, mea- 

 suring in one specimen thirty inches by twelve, with a cover- 

 ing of strong plates, wavily grooved and overlapping each 

 other, the head forming only a slight rounded projection from 

 the general figure. We here find another early and startling 

 example, in addition to the brontes, of animals which may be 

 called large. In the strata of this formation at Db'rpat, there 

 are gigantic bones, which were at first thought to belong to 

 reptiles, but have since been ascertained to be remains of 

 fishes, leading to the conjecture that the animals to which 

 they appertained could not be less than thirty- six feet 

 long (i9). 



M. Agassiz has lately announced nine genera of sharks of 

 the division Cestraceon in the Devonians of Russia. It is in 

 this voracious family that we see the placoids represented in 

 modern seas ; the ganoids are all but unrepresented in our 

 time. Of both classes, one invariable peculiarity has at- 

 tracted much attention. " In all recent fish, with the excep- 

 tion of the shark family, the sturgeon, and the bony pike, the 

 vertebral column terminates at the point where the caudal fin 

 is given off, and this fin is expanded above and below the 

 body, forming what is called a homocercal tail. In all those, 

 without exception, which have been found in strata of the 

 Palaeozoic period, [placoids and ganoids,] the caudal fin is he- 

 terocercal) being formed of two unequal branches, the upper 



