748 THE PALEONTOLOGY OF MINNESOTA. 



[Platymotopus blcornls. 



subgenus Platymetopus. I regret being unable to concur with Mr. Ulrich's reference 

 of this and the following species to Dames' proposed subdivision Hoplolichas, but in 

 the latter the third lobes are well defined. Dames ascribed much importance to, 

 and indeed, found the suggestion of his term in the stout, sometimes forked spine 

 borne by the occipital ring,* while the possession of anterior extensions of the frontal 

 lobe, though of much the same significance structurally, is taxonomically unessential. 

 It would seem, in fact, that if there is any basis for the admission of the division 

 Hoplolichas, it lies in the presence of this ornamental or defensive character. The 

 original specimen of P. robbinsi is broken near the center of the occipital ring but 

 there is no indication that it possessed a central nuchal spine. 



In the possession by different subgenera of Lichas, of similar frontal extensions 

 of the glabella, as in L. (Metopias) pachyrhyncha, var. longirostrata Schmidt, L. 

 (Hoplolichas) proboscidea Dames and L. (Platymetopus) robbinsi Ulricb, we find an 

 instance of morphic equivalence in a certain structural character coexisting with 

 subgeneric features essentially distinct. 



Formation and locality. Platymetopus robbinsi is from the middle beds of the Galena limestone, at 

 Wykofl, Minnesota. (Collection of Mr. E. O. Ulrich). 



PLATYMBTOPUS BICORNIS Ulrich, (sp.), 1892. 



Lichut (Hoplolichas) bicornis ULRIOH, 1892. Two new Lower Silurian species of Lichas (Subgenus 

 Hoplolichas); Amer. Geologist, vol. x. p. 272, flgs. 2a-6. 



This interesting species has precisely the same character of glabellar lobation 

 as the preceding, and the remarks made upon the generic relation of the former 

 apply as well to this. In the possession of a pair of divergent spines on the frontal 

 lobe it would seem to bear a similar relation to Hoplolichas tricuspidata Beyrich, as 

 P. robbinsi does to H. proboscidea. The characters of the species, as far as known 

 from a single cranidium, have been sufficiently described by Mr. Ulrich, and will be 

 apparent from the accompanying figures. 



Figs. 70, 71. Cranidium of Platytnetopus bicornis Ulrich, with outline profile. 



Formation and loality. Hudson River group; two miles east of Spring Valley, Minnesota. (Collection 

 of Mr. E. O. Ulrich). 



See liiiincs. /.-Itschr. d. deutecb. geolog. Oesellsch.. vol. xxix., p. 71)4. pi. 12-14, 1877. 



