ON A NEW GENUS AND SPECIES OF HLASTOIDS. 87 



the two Others the summit scarcely reaches the level of the peris- 

 tome; it also has narrower, almost linear ambulacra, against slightly 

 petaloid ones in the others, and these are divided by the food-groove 

 only; while all T>ouisville species have three indented lines along their 

 surface. It further differs from H. alternatum in having straight, and 

 not convex, sides along the calyx, and a conical, more sharply pointed 

 basal cup. 



It is barely possible that Peiiiremites siihtnincatiis, Hall (Geol. Rep. 

 Iowa, Vol. I., Pt. II., p. 485), is identical with this species. A com- 

 parison is im])ossible, as Hall's description is so indistinct that not 

 even the genus can be identified. He refers it to forms like Pentre- 

 mites R/ieinwardti, and mentions "ovarian openings," which, if present, 

 suggest a very different thing. Should, however, hereafter, the identity 

 be proved, the name must be i:)laced as a synonym under Heteroschisma 

 gracile. 



Position and locality, etc.: Found near the top of the Hamilton 

 group, Alpena, Michigan. 



[The type specimens are in the collections of Prof. Barris and 

 Charles Wachsmuth.] 



