630 THE AMERICAN NATURALIST [Vol. XLII 



armored dinosaurs from the Hell Creek Beds of Montana. The 

 material, however, upon which he bases his restoration was 

 scanty — too scanty to serve as a satisfactory basis for a restora- 

 tion — consisting of the skull, a number of vertebrae, a somewhat 

 problematical scapula, and a number of dermal scutes. The 

 dangers of such attempted restorations as the present one on 

 such imperfect material are apparent here, since Ankylosaurus 

 is either very closely allied to or identical with Stegopelta 

 Williston, a genus overlooked by Mr. Brown. And Stegopelta, 

 which is represented by considerable material in the University 

 of Chicago collections, is as closely allied as well may be with 

 Polacanthus Iliiike of Kngland, and must go in the same family. 

 A comparison of ■ Nopsea 's restoration of Polacanthus, which is 

 essentially correct, save for the skull, will show the real form 

 of Ankylosaurus, very different from that given by Mr. Brown. 

 Stegopelta, moreover, has spines, and the body is covered in 

 large part by small scutes, though there are also large ones, as 

 figured by Mr. Brown; and the tail rings are correctly placed 

 by Brown. The writer has examined the specimen in the British 

 Museum, and is assured of the relationships. Stegopelta will 

 shortly be more fully described and figured by Dr. Moodie. 



S. W. Williston. 



The Evolution of Parasitism.— Tn a recent paper entitled: "The 

 Influence of Symbiosis upon the Pathogenicity of Microorg— 

 isms (The Evolution of Parasitism)," Musgrave 1 has bro - 

 together some most important items in an extremely suggesti 

 manner. He defines symbiosis as representing all phases of asso- 

 ciation between living organisms, beginning with commensalism, 

 on the one hand, and including true parasitism, on the other, in 

 which either component is influenced in nutrition, metabolism, 

 production or in some other manner by the presence of the other. 

 While this use of the term is distinctly new and rather at vari- 

 ance with the older usage of van Beneden the meaning of the 



