AMERICAN PALEOZOIC CEPHALOPODS 
225 
23. DAWSONOCERAS HYATT 
Genotype: Orthoceras annulatum of McGill college, Montreal 
Dawsonoceras is another instance of a genus erected by Hyatt 
chiefly with Biarande species in mind, but with an American 
species designated as the type. In his original description he 
states that the genus includes forms like Orthoceras pseudoca- 
lamiteum Barrande, but at the close of his description he desig- 
nates Orthoceras annulatum, from the Museum at McGill College, 
as the type. The latter belongs to the species characterized by 
transverse annulations and transverse frilled striae. Longi- 
tudinal ridges, considerably less conspicuous than the transverse 
annulations, may or may not be present. 
In the Zittel-Eastman Text-book of Paleontology Hyatt figures 
under Dawsonoceras the Orthoceras annulatum of Bohemia, af- 
ter Barrande, and not the specimen in the Museum of McGill 
College. It is evident, from the presence of frilled transveise 
striae, that Orthoceras amycus Hall belongs in the genus Daiv- 
sonoceras. 
In Perigrammoceras the transverse striae are straight and not 
frilled. 
24. SPYROCERAS HYATT 
Genotype: Orthoceras crotalum Hall. Pal. New York, vol. 5, 
pt. 2, 1879, pis. 42, 82, 113 
In defining this genus Hyatt apparently had in mind chiefly 
groups 5 and 6 of the species of Orthoceras described by Bar- 
rande, but, when it came to selecting the genotype, he chose Or- 
thoceras crotalum Hall from the Hamilton formation of New 
York. 
The conch of Orthoceras crotalum is crossed transversely by 
strong, sharply defined, and rather distant annulations. The 
vertical striae are very fine and closely crowded. A similar 
species, with closely crowded vertical striae, but with wider and 
less sharply elevated annulations, is found in the Richmond of 
Anticosti. 
In general, however, the earlier species of Spyroceras, in the 
Ordovician, have stronger and less crowded vertical markings, 
often designated as ribs, or as ribs alternating with vertical 
striae. These may form a group distinct from the finely striated 
forms. 
