208 BULLETIN : MUSEUM 
OF COMPARATIVE ZOOLOGY. 
Physonemus arcuatus M’Coy. 
(Text-figure 12.) 
1848. Physonemus arcuatus F. M’Coy, Ann. Mag. Nat. Hist. (2), Vol. IL, p. 117. 
1855. Physonemus arcuatus F. M’Coy, 
Brit. Palaeoz. Foss., p. 638, Pl. III, Fig. 29. 
1875. Drepanacanthus reversus St. John and Worthen, Pal. Illinois, Vol. VI., p. 456, 
Pl. XIX., Fig. 5 (non Fig. 6). 
Fie. 12. 
Physonemus arcuatus M’Coy. St. 
Louis limestone, Alton, Ill. Lat- 
eral aspect of spine, X 4}, and 
single denticle, < +. 
1883. Physonemus arcuatus J. W. Davis, 
Trans. Roy. Dublin Soe. (2), Vol. I., 
p. 367, Pl. XLVIL., Fig. 8. 
1883. Physonemus falcatus St. John and 
Worthen, Pal. Illinois, Vol. VIL, 
p: 252, Pl XXIV. Fie Ge 
1885. Drepanacanthus reversus St. John and 
Worthen, Jbid., p. 258, Pl. XXIV., 
Fig. 5. 
1899. Physonemus stellatus J. S. Newberry, 
Monogr. U. S. Geol. Surv., Vol. 
XVI., p: 200; Pl. XXIe Bigeag: 
1902. Physonemus arcuatus C. R. Eastman, 
Bull. Mus. Comp. Zool., Vol. 
XXXIX., p. 87. 
All of the spines referable to this species 
which have been previously figured are im- 
perfect in this respect, that the superficial 
ornamentation has been very largely de- 
nuded, and the denticles along the concave 
margin either worn or broken away, thus 
obscuring their true relations. Although 
the example shown in the adjoining Text- 
figure has been somewhat injured, its gen- 
eral outline is well displayed, and enough 
of the ornamentation remains to leave no 
doubt as to its identity with the type 
species of this genus. The double row of 
striated denticles bordering the concave 
margin is fully as prominent as in typical 
examples, although abrasion has reduced 
many of the tubercles in size. One of the 
latter is represented seven times the natu- 
ral size in the figure to the left of the spine. 
Owing to the worn condition of the basal portion, it is not apparent to what 
depth the spine was inserted in the integument, but from some other speci- 
mens the writer has seen it is probable that it was not deeply implanted. There 
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