210 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
noticed its resemblance in ornamentation to that of P. gemmatus from the same 
horizon. The specimens figured by St. John and Worthen are likewise from 
the distal third of the spine. The total length, as shown by one or two good 
examples in Iowa City, is really much longer than these authors supposed, and 
the breadth nearly equals that of P. gemmatus. The proportions of the two 
species are in fact very similar, the chief differences consisting in ornamental 
details, form of cross-section, and the greater development of tubercles along 
the concave border in the present species. In the distal portion of the spine 
these tubercles frequently appear as strong acuminate denticles. The pulp- 
cavity remains open for a short distance on the convex side near the base, and 
continues close to this side throughout the spine until near the tip. Like the 
preceding species, it is extremely probable that the spines of P. stellatus were 
curved forwards. 
Formation and Locality. — Keokuk limestone ; Iowa and Illinois. 
LIST OF NORTH AMERICAN SPECIES OF PHYSONEMUS. 
: od 
g § oi oi 
Name of Species. 3 = = z § 5 
SB [-8 3) 2 Seales 
i a) i a s) OF 
1. P. hamus-piscatorius Eastman . X —,| — | — | — — 
2...“ “pandatus Bastman..¢.) 0s +) x _ — —}— — 
3. “ gigas Newb. and Worth .. . — X me | — 
4. “ gemmatus (Newb. and Worth.) . — | — ya ear iy — 
5. “ stellatus (Newb. and Worth.) . . — — X — | — = 
6. “ (2) baculiformis (St. J.and Worth.) — — X — | — — 
7. % (2) nects (St. d.and Werth.) 2 = a _ X — | — — 
8. “rarenaius MOCOy'. «a os — — | — X — — 
9. “ acinaciformis (St. J.and Worth.) | — —}|—|]—|{—- X 
10. “ anceps (Newb. and Worth.) . . _ —}—}|—]— 
LS nasper Bastian.) %: rasa i o — — —}|— xX 
| | 
ERISMACANTHUS MCoy. 
The two European and, one American species of this genus that have been 
described are evidently very closely related to the type of Physonemus, but 
differ in that the spines are divaricated, the two branches extending in opposite 
directions in the same vertical plane. The imperfect Ichthyodorulites known 
