132 TRANSACTIONS OF THE [MAR. 10, 



bevelled and acute. Most dorsal spines of sharks found fossil 

 have a conical, unornamented, solid base which was planted to 

 a greater or less depth into the integument of the back, as a post 

 is sunk in the ground; but the spines of Oracanthus show no- 

 thing of this solid base, and have the appearance of having been 

 attached to the back as barnacles are fastened to the skin of the 

 whale. This would seem to be a very inadequate support for 

 these monstrous spines, and would make them only ornaments 

 and not weapons. Hence their true structure and uses have 

 been up to the present time a mystery. I have, however, re- 

 cently received from Mr. W. F. E. Gurley, of Danville, 111., a 

 complete spine of Oracanthus veUistus Leidy, which fully 

 solves the problem. Of this spine the exposed portion is a flat- 

 tened cone seven inches high by five inches broad at the base; 

 below this the buried portion is only about half an inch wide, 

 but this is prolonged anteriorly into a transversely arched root 

 or base about six inches long, making the entire antero-posterior 

 breadth of the base more than ten inches. This shows that the 

 spine was curved anteriorly and was sujjported by a strong root 

 or base thrown out forward, which must have held it so firmly 

 as to resist all blows or pressure from before. The surface of 

 the exposed portion is covered with a sheet of enamel thickly 

 set with conical, often stellate, tubercles. As it is much com- 

 pressed and the anterior margin somewhat acute, the summit 

 being curved forward, it forms a sickle-like organ, which, firmly 

 rooted in the manner described, must have been a formidable 

 weapon. 



Such spines as that described above are quite symmetrica], 

 and must have been placed on the median line of the back, but 

 in the same beds with these are found short, broadly triangular 

 spines which have the surface markings of Oracanthus, yet are 

 so different that they have been with doubt referred to this 

 genus. Several small spines of this peculiar character which 

 occur in the Corniferous limestone of Ohio, I described in the 

 Biilleti/i of the National Institute for lb57 under the names 

 of Oracanthus fragilis, 0. granulatns, 0. abbreviatus, and 0. 

 muUiseriatus. Later (1866) I described a similar spine from 

 the Keokuk limestone in the second volume of the " Geological 

 Survey of Illinois" (p. 117, pi. xii., fig. 3) with the name of 0. 

 pnigeus. In 1875 Mr. Orestes St. John took this latter spine 

 as the type of a new genus, to which he gave the wKmeoiPnige- 

 acanthus (''Geological Survey of Illinois," Vol. VI., p. 480), 

 giving to it the specific name of P. deltoides. 



Facts which have recently come to my knowledge lead me to 

 believe that all these last-mentioned short, broad, thin-shelled 

 spines truly belong to the genus Oracanthus and were the de- 



