120 Relations of Edestiis. 



teries, as sliown in tigs. 'Z b of PI. V. I also have from Ver- 

 million Co., Indiana, a specimen lignred on PL V fig. 2 a, 

 Avhich seems to be tlie basal segment of a spine, probably of a 

 yonng individual, of Edestios Heiiirichsii. This is a spatnlate 

 solid bone carrying a beantifully perfect enameled denticle at its 

 extremity. The shells or sheaths obtained from Mr. Butters are 

 similar to this, except that each one is a trough into which the 

 succeeding one fits, and the added cap covers a portion of the 

 enameled base of its predecessor. If this is all true, and it 

 seems undeniable, we are compelled to conclude thas the spine 

 was buried in the integuments throughout its entire length; 

 the enameled denticles alone projecting above the surface to 

 form a saw which would be a terrible weapon, if placed upon 

 some flexible portion of the body where it could be used with 

 f]-eedom and power. The extremity of the spine may have lain 

 in a sheath from which it could be partially erected by muscular 

 action, and used as the lancet of the surgeon fish (Acanthurus) 

 is ; but the bilatei'al symmetry of Edestus proves that if em- 

 ployed in this manner it must have been located on the upper 

 margin of the tail oi'back. 



The segmented stiucture of Edestus has led Dr. Woodward to 

 compare it with the segmented spines of Pelecopteriis, and es- 

 pecially with the ])ectoral spines of this genus described by Prof. 

 E. D. Cope, (Geol. Survey of the Territories, Vol. II, ]). 244 

 A)., but the symmetry of Edestus forbids the acceptance of this 

 conclusion. The i)ectoral spines of all fishes are unsymmetrical. 

 This is plainly seen in MachcBracantlius and Gyracarithus, and, 

 as I have lately shown, in the pectoral spines of Stetliacantlms 

 {Pliy son emus) Altonensis. Pelecoj)terus probably had dorsal 

 as well as pectoral spines, and a comparison with them would be 

 better grounded ; but as that was a bony fish, the dorsal spines 

 would have an articulation at base, and would have grown at the 

 base and not at the summit. 



In the spines of Trygon, however, we find a much closer re- 

 semblance to Edestus ; one that seems to me to go far towards 

 solving the problem of the relations and functions of these j^e- 

 culiar organs, and almost decides that they are dorsal spines. 

 In Trygon a considerable number, sometimes five or six, defen- 

 sive spines are set in the place of the posterior dorsal fin. They 

 come into use in succession, like the fangs of venomous serpents. 



