550 TRANS. SI . LOUIS ACAD. SCIENCE. 



cral and interambulacral, which are less in number and not so 

 regularly arranged. The general size of the body varies from 

 if to 2^ inches and its transverse diameter is i larger than the 

 vertical one. 



Geological formation and locality — In the upper St. Louis lime- 

 stone, St. Louis, Missouri. 



Genus Oligoporus Meek and Worthen. 

 Oligoporus parvus, n. s, 



(PLC, Fig. 3) 



From this species I possess only crushed, or rather imperfect 

 specimens, like all others I have seen ; however, some parts are 

 sufficiently preserved so as to allow a correct idea of the form of 

 the body, which was undoubtedly similar to the others of the 

 same genus. The ambulacra are narrow, running over the whole 

 surface of the body, and are composed of four rows of poral 

 plates, very irregular in size as well as form, of which the larger 

 ones form in the centre of the field quite an elevated ridge, 

 with here and there an odd plate inserted between the regular 

 rows ; each plate is perforated by two pores near the outer 

 margin of the plate, thus forming two double rows of pores to 

 each side of the ridge. Interambulacral space nearly one-third 

 wider than the ambulacral, and composed of hexagonal and pen- 

 tagonal plates very coarsely granulated and covered with very 

 short spines; the number of plates is not over five in the greatest 

 width of the field. Oral and genital plates not visible. 



Geological formation and locality — In the lower St. Louis lime- 

 stone, St. Louis, Missouri. Specimen in the collection of the 

 Washington University. 



Making due allowance for the crushed condition in which I 

 have found all the specimens I have seen, I think that the follow- 

 ing measurement will give as near as possible the actual size of 

 the body : transverse diameter, about 3 inches ; vertical height, 

 about if inches. 



