NO. 12 SPENCE SHALE AND ITS FAUNA RESSER 9 



ZACANTHOIDEA Swinnerton 



ZACANTHOIDES Walcott, 1888 



ZACANTHOIDES IDAHOENSIS Walcott 



Plate 3, figs. 18-20 



Zacanthoidcs idahocnsis Walcott (part), Smithsonian Misc. Coll., vol. 53, 

 no. 2, p. 26, pi. 3, figs. I, 6, 1908. [Not figs. 2-5, io = Z. Jiolopygus; figs. 

 / ^ Z. disjuucfus; figs. 8, 9, 11 = Z. gradaiits.] 



This species is now restricted by elimination of three other species 

 included among the original illustrations. Walcott mentioned the 

 different forms present but did not carry his observations to their 

 logical conclusion. 



Locality 55c. 



Lcctotypc. — U.S.N.M. no. 53434; paratypes, nos. 53432, 53438. 



ZACANTHOIDES ADJUNCTUS, n. sp. 



Plate 3, figs. 13, 14 



Zacmithoidcs idahocnsis Walcott (part), Smithsonian Misc. Coll., vol. 53, no. 2, 

 p. 26, pi. 3, fig. 7, 1908. (See Z. idahocnsis.) 



Walcott figured a pygidium of this species. The pygidium of Z. 

 adjunctus is characterized by a wide axis and extensive fusion of the 

 pleura, so that the tail makes a far more solid plate than in Z. 

 idahocnsis. The outer spine is long but is free only as far as the end 

 of the axis. The second pair of spines is fairly long, but slender, 

 and the remaining three pairs of spines are short and sharp-pointed. 



Locality 55c. 



Holotype and paratypes. — U.S.N.M. nos. 53429, 53427. 



ZACANTHOIDES GRADATUS, n. sp. 



Plate 3, figs. 15-17 



Zacanthoidcs idahocnsis Walcott (part), Smithsonian Misc. Coll., vol. 53, no. 2, 

 p. 26, pi. 3, figs. 8, 9, II, 1908. (See Z. idahocnsis.) 



This species is characterized by a rather broad pygidial axis. The 

 marginal spines decrease in size slightly for the first three pairs; then 

 abruptly for the remainder. It is thus that the species is distinguished 

 chiefly from Z. idahocnsis in which the pygidial spines continue to 

 decrease at a fairly even rate from the outer to the inner pair. 



The cranidium is referred to the species on the basis of its wide 

 glabella. 



Locality 55c. 



Holotype. — L^S.N.M. no. 96521 ; paratypes, nos. 53430-1, 3. 



