C. D. Walcott — Position of the Olenellus Fauna. 37 



rotomaria Attleborensis does not appear to have a representa- 

 tive before reaching the species of the Lower Ordovician fauna, 

 and Straparollina remota has no connection through the known 

 Middle Cambrian fauna with the fauna of the Ordovician. 



Pteropoda. — The four genera and fifteen species of this 

 close are very strongly related to those of the Middle Cam- 

 brian fauna. Ilyolithes princeps is a large form that is very 

 abundant in the Olenellus zone, and it has a great geographical 

 range. It is found in Western Nevada, Eastern Massachusetts 

 and Eastern Newfoundland. A closely allied if not identical 

 species occurs in the Paradoxides zone in Newfoundland. H. 

 maximus of the Paradoxides zone of Bohemia is of this same 

 specific type, although diifering considerably in detail of form. 

 H. Americanus is very closely allied in form to II. Acadica 

 of the Paradoxides zone in New Brunswick, and the same 

 type is abundant in the Upper Cambrian of the Mississippi 

 Yalley, under the name of H. primordialis. H. Billingsi is 

 known to range from the Lower to the Middle Cambrian, and 

 has been found in Labrador, New York and Nevada. H. com- 

 munis, H. impar, H. quadricostatus and H. Terranomcus are 

 species which do not appear to have representatives in the 

 Middle Cambrian fauna. H. similis is very much like H. 

 primus of the Paradoxides zone of Bohemia. The genera 

 Hyolithellus and Coleoloides do not appear to be represented 

 by well authenticated species in the Middle Cambrian. Sal- 

 terella is not met with again until the Ordovician fauna is 

 reached and there very doubtfully. 



Crustacea. — Of the true Crustaceans, Leper ditia derma- 

 toides has a close specific relationship with an undescribed 

 species from the Middle Cambrian of the Grand Canon section 

 of Arizona; and a representative species, Z.» Stissingensis, 

 occurs in the New York Middle Cambrian. 



The genus Aristozoa, although abundantly represented in 

 the Silurian fauna of Europe, is not known from the Middle 

 Cambrian fauna. Protocaris Marshi still remains the oldest 

 known Phyllopod crustacean. The Upper Cambrian Phyllo- 

 pod, Hymenocaris vermicauda, is the next met with, unless 

 some of the forms referred to Stenotheca, in the Paradoxides 

 zone, are portions of the carapace of some species whose gen- 

 eric relations are undetermined. 



Trilobita. — The sixteen genera and fifty-three species of tri- 

 lobites constitute less than one-third of the entire fauna. The 

 range of variation among the genera and species includes forms 

 with and without eyes, and with and without facial sutures. 

 One of the surprising facts is that the genus Agnostus, which 

 has been theoretically considered the ancestral form of the 

 trilobite, does not appear to exist in the lower portion of the 



