lO SMITTTSONfAN MISCELLANEOUS COLLECTIONS VOL. 97 



ZACANTH0IDE5 ABBREVIATUS, n. sp. 



Plate 2, figs. 7-9 



Z. abbrcviatiis is represented by fewer specimens than most other 

 species in the Spence shale. It is characterized by a rather wide axis 

 and a considerable degree of fusion in the pygidium. Like Z. ida- 

 Jioensis the pygidial spines decrease in size at a regular rate from the 

 outer to the inner pair. But these spines are shorter, the axis stouter 

 and the degree of fusion of the pygidium greater than in Z. idahocnsis. 



Locality 5Sc. 



Holotype and paratypcs. — U.S.N.M. no. 96501. 



ZACANTHOIDES SERRATUS, n. sp. 



Plate 2, figs. 4-6 



This small species is relatively uncommon. Walcott had illustra- 

 tions of this distinctive pygidium prepared, but recognizing it as a 

 separate species, did not publish the figures. Aside from the slender 

 axis throughout, the rather long thoracic pleura and the pygidial struc- 

 ture characterize the species. In it the spines are fused but remain 

 distinct, terminating in rather broad short spines. These spines termi- 

 nate at nearly a straight line, hence give the rear of the pygidium 

 its serrate margin. 



Locality 55c. 



Holotype and paratypcs. — U.S.N.M. no. 96502. 



ZACANTHOIDES HOLOPYGUS, n. sp. 



Plate 2, figs. 10-12 



Zacanthoides idahoensis Walcott (part), Smithsonian Misc. Coll., vol. 53, no. 2, 

 p. 26, pi. 3, figs. 2-5, ID, 1908. (See Z. idahoensis.) 



At first it was thought that this small species was merely a young 

 stage of one of the larger forms, but careful sorting shows that such 

 is not the case. Z. holopygus varies in length from less than one- 

 eighth of an inch to more than 2 inches. As a whole this trilobite 

 has a more even oval shape than most species of Zacanthoides, which 

 is due to the fact that the thoracic terminations are relatively broader. 

 It is the most common Spence shale species. 



Z. holopygus has rather large eyes, and the anterior facial suture 

 diverges sharply, leaving rather long anterior angles. The pygidium 

 is fused into a solid shield, including all marginal spines except the 



