262 Nezv York State Museum 



in numbers of species from those now known to us from 

 the Cambrian and even the Ozarkian, despite more limited 

 exposures and less favorable rocks for preservation, is 

 already crowding the Ordovician in numbers of species. 

 The Canadian has many, yet fewer than any of the other 

 Eopaleozoic systems (Ulrich). More than 300 forms are 

 known from the Ordovician, and they belong for the most 

 part to intermediate types. Disregarding the giants, the 

 Ordovician trilobites differ little in average size from 

 those of other systems. Few of the lowest types have 

 survived and few of the highest types have been intro- 

 duced. Among the most striking forms is Isotelus in 

 which the head and tail are similar and each constitutes 

 about one-third the length. It is as large as any trilobite 

 known, reaching 24 inches in length and ten inches or 

 more in width. Illaenus is as abundant and new for the 

 period as any. Other characteristic genera are Calymene, 

 which is often found coiled in a ball, the spiny Ceraurus, 

 Bumastus and Trinucleus. Trinucleas (Cryptolithus), 

 distinguished by a large head with three bulging lobes and 

 a flat, ornamented rim prolonged into a lateral spine on 

 each side, is one of the most characteristic forms. Ostra- 

 cods, small; bivalve crustaceans, were also very abundant 

 during all or most portions of the period. One of the 

 common genera is Leperditia. Another group is repre- 

 sented in the Ordovician, but not abundantly. These are 

 the eurypterids which had a remarkable development in 

 the Silurian and Devonian. The existence of vertebrates 

 in the Ordovician is doubtful. The fish remains found 

 in the basal sandstone (Harding) of the Rocky Mountain 

 region (Colorado and Wyoming) are of Silurian age. 

 The Harding sandstone is much younger than the St 



