532 GEOLOGY OF THE YELLOWSTONE NATIONAL PARK. 



spine bases shaped like small rose thorns, tapering at either end. In the 

 specimens which I have examined (none of them, it may be noticed, quite 

 attain the size shown in the figure) the posterior portion alone presents this 

 appearance, and even there the spine bases are so long as to resemble 

 discontinuous stria;. They taper only in one direction anteriorly, and end 

 suddenly and acutely, looking like slender spines, appressed or in demi- 

 relief. The termination, however, itself formed the base for a small spine. 

 These ridges then are longer, more distant, and differently shaped from the 

 figure. The ridges grow stronger, broader, and more continuous as the 

 shell increases in size, so that the whole anterior margin is regularly and 

 strongly- marked with spine-bearing striae, from ten to twelve in 10 mm. 

 I do not know the condition of Hall's types; but I have noticed that in 

 localities such as Burbank, where the shell substance has been leached 

 away, leaving casts in the sandstone, these are apt to show the features 

 represented in the figure. 



The surface ornamentation of P. scabriculus is similar to that figured 

 for P. newbernji, consisting of fine, numerous, elongated, spiniferous pus- 

 tules, the spines being situated near the anterior end; and it is also very 

 similar to, though somewhat finer than, that of P. pa/pulata of the Hamilton 

 group. This is, however, different from the Waverly specimens of P. neiv- 

 berryi, and the shape is also different, being more highly arched, less spread- 

 ing, and with a more produced incurved beak. In shape and surface this 

 form is very similar to P. scabriculus from the Carboniferous of Glasgow, 

 Scotland, the chief difference arising from the fact that in the latter the 

 pustules are somewhat more coarsely distributed and show conspicuous, 

 though irregular, inequalities in size. 



The dorsal valve of this species, as seen at a different locality from the 

 specimen figured, when viewed from the convex side, is crossed by numer- 

 ous fine concentric wrinkles, and is covered with fine pustules, smaller and 

 more closely arranged than those of the other valve. The casts of small 

 spines can be seen among the pustules The ventral valves associated with 

 these are of the normal type. 



I have retained the collected synonymy of this species, although I 

 doubt whether any of the citations are really identical with the form under 

 discussion or with P. scabriculus. P scabriculus, it should be noticed, is in 

 Europe a Devonian and Lower Carboniferous form, while all or nearly all 



