260 New York State Museum 



stars and sea urchins, but the blastoids and sea urchins 

 are extremely rare, the blastoids being represented by 

 only one species and that somewhat doubtful. Representa- 

 tives of the last three groups are rare but very character- 

 istic when found. Cystoids, as pointed out above, reached 

 their culmination in this period. Among the character- 

 istic and common genera are Malocystis, Pleurocystis 

 and Agelacrinus. The earliest blastoids (Blastoido- 

 criniis) appear in the Lower Ordovician (Chazy), but 

 they became important much later (Carboniferous). 

 These primitive forms show notable cystoidian charac- 

 ters. The crinoids were locally so abundant in the Ordo- 

 vician that in certain places whole layers of rocks are 

 composed of their dissociated plates. Even heads are 

 abundant. Among the common genera are Glyptocrinus, 

 Heterocrinus, Dendrocrinus etc. While crinoids were 

 numerous in the Ordovician, they did not attain their 

 greatest development until later. Bryozoans were so 

 numerous that they were more important as limestone 

 makers than the brachiopods. They were abundant not 

 only in numbers but in species, probably numbering more 

 than a thousand, and are valuable as index fossils because 

 of their great number, wide distribution and limited range 

 in time. Brachiopods, too, are found very abundantly in 

 Ordovician beds and are also important as horizon mark- 

 ers. About 500 forms are known for North America. 

 The thin-shelled hingeless types so characteristic of 

 earlier periods are less in evidence, and have been re- 

 placed by the hinged forms, though the highest types have 

 not yet made their appearance in numbers. Among the 

 genera found are Lingula, Schizocrania, Rafinesquina and 

 the reversed form Strophomena, Dahnanella, Dinorthis, 

 Platystrophia, Zygospira, Rhynchotrema, Camarotoechia 



