ECHINODERMATA—BLASTOIDEA. 477 



f. Base usually forming a deep cup XL Cadaster 



a. Summit contracted ** 



**. Posterior spiracles confluent with anus II. "Froostocrititis, 



■'^*. Posterior spiracles separate from anus III. Metablastus. 



1. Greatest diameter of calyx central or below VI. Pentremites. 



B. Base not tapering ; calyx globular or ovoidal 2, 



2. Deltoids very broad, resembling sharks' teeth I. Blastoidocrinus. 



2. Deltoids small, or if long, narrow b 



b. Ambulacra linear, extending nearly or quite the whole length of the calyx 



*** 



***. Spiracles in pairs ff 



•ff . Posterior deltoid divided into two parts by an anal plate. 



VII. Eleacrimis. 



ff. No anal plate present i^ 



\' . Hydrospire pores present along the edges of the deltoids. 



VIII. Schizoblastus. 

 i^. No hydrospire pores present along the deltoids..IX. Cryptoblastus. 



**^. Spiracles five, at the ends of the deltoids X. Granatocrinus. 



b. Ambulacra narrow, not extending the whole length of the calyx *-x-** 



****. Spiracles five V. Pentremitidea. 



****. Spiracles double IV. Tricoelocrinus. 



b. Ambulacra broad and petaloid, extending half way or more down the calyx. 



VI. Pentremites. 



I. Blastoidocrinus Billings. 



Rarely found entire. Like Pentrcnvites in general form. Base 

 deeply invaginated, appearing in side view as a low inverted trun- 

 cate cone. Greatest diameter about one fifth vertical distance 

 from bottom, whence the calyx slopes regularly upward in a low 

 dome. Basals unknown but probably very small. Radials five, 

 many-angled and bent, forming below a deep conical pit with outer 

 rim about twice the diameter of the stem and surrounding the 

 stem for a distance of seven or more rings. Each radial orna- 

 mented with a mound on the rim of the pit whence radiate upward 

 10 or 20 depressed grooves. Each radial joined above to two 

 irregular bibrachial plates, with many smaller interradial plates 

 between. The dome-shaped upper portion of the calyx consists 

 chiefly of the five great triangular concave deltoid plates, super- 

 ficially resembling sharks' teeth. Ambulacral areas between the 

 deltoids occupied by slender, pavement-like plates (brachioles), 

 with a series of three wing plates down the center of each area. 

 Central apical plate star-shaped. 



One specimen may consist of 50,000 plates and ossicles. 

 Ordovicic. 



