CLARK: NEW BLASTOIDS AND BRACHIOPODS. 363 



within a given species may tend to detract from the advantage of this 

 system of tabulating the characteristics of Pentremites, but, on the 

 whole, I feel confident that such a system will be of great helj) in 

 separating the species of this genus. 



For Schizohlastus, I have used a standard of ten millimeters, fearing 

 lest further magnification should nuiltiply unavoidable errors in 

 measurements. For larger specimens than ours, twenty millimeters 

 may prove to be better. 



The following pages contain a list of all blastoids which have, to the 

 knowledge of the writer, been reported from the Rocky Mountains. 



Pentremites saxiomontanus, sp. nov. 



Plate 1, fig. 1-6, 14. 



Description. — Body of medium size, ovate, thickest at the base of 

 the ambulacra. Basal portion obconical, prominent, occupying about 

 a quarter of the length of the specimen. The radial plates are nar- 

 row, each with a markedly prominent angular median ridge below the 

 bifurcation; that part of the radials between the ridges Ijeing perfectly 

 flat. " Each fork of the radials is slightly convex, thus making a 

 median depression along each interambulacral area. The radials 

 are ornamented with fine striae running parallel to the suture, ending 

 against the ambulacral border, and apparently converging slightly 

 towards the base. Deltoids very small, unornamented, short, and 

 narrow. The interambulacral area shows only a slightly raised 

 border, in some specimens none at all. The ambulacra extend to the 

 thickest part of the body, about three quarters the entire length, 

 enlarging constantly to near the summit; hence the sides are not 

 subparallel. 



Dimensions of three specimens ^ in milliineters. 



Height 13. 14. 15. 



Maximum diameter 9. 10. 12. 



Length of ambulacral plates 9. 9.5 10.5 



Maximum witlth of ambulacral area... 3.8 3.5 4. 



Length of deltoid plates 2.5 3.5 3. 



Maximum width of deltoid plates 1.5 1.5 1.5 



Average numl)er of side-plates in 5 mm.. 13. 16. 13. 



' These three specimens are figured on l'L<ti> 1. in onler, fig. 4, 1, and 14. 



