218 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
Homacanthus delicatulus, sp. nov. 
(Plate 3, Fig. 28; Plate 5, Fig. 59.) 
Type. — Isolated spine; Museum of Comparative Zoology, Cambridge. 
Definition. —Spines very diminutive, erect, deeply inserted; base of ex- 
serted portion relatively broad, distal extremity acute, sides ornamented with 
not more than five or six straight longitudinal ridges. 
The very minute and nearly perfect specimen which is here referred to 
Homacanthus might at first sight be assumed to belong to a young individual 
of Ctenacanthus, in which case it would correspond to the extreme tip of an 
adult spine. But even the distal extremity of all Ctenacanthus spines is 
distinctly tuberculated, and moreover, the costae appear too divergent, and the 
base of insertion too broad for this specimen to be regarded as a young form of 
Ctenacanthus. Besides, the Kinderhook species of Ctenacanthus are pretty 
well known, and there are none to which this small form corresponds even 
approximately, hence we may look upon it as belonging to Homacanthus. 
The spine has a total length of about 1.8 cm., and maximum width of only 
3mm. The inserted portion is relatively very long and tapering, and the ex- 
serted portion has a narrow triangular form, the two portions being separated 
by a very oblique and prominently marked line of insertion. The lateral face is 
occupied by five smooth and continuous longitudinal costae, and two or three 
additional ones unite to form the anterior keel. Growth of the costae seems 
to have taken place by the coalescence of dentine tubercles formed just below 
the line of insertion, as shown in Plate 5, Fig. 59. The absence of denticles 
along the posterior margin is to be accounted for by the effects of weathering 
or abrasion, or both. Some resemblance is to be noted between this spine and 
one of those figured by J. W. Davis as H. microdus from the Lower Carbon- 
iferous limestone of Armagh, Ireland (Trans. Roy. Dublin Soc. (2), Vol. L., 
1883, Pl. XLVIII., Fig. 8.) 
Formation and Locality. — Kinderhook limestone ; Le Grand, Iowa. 
Homacanthus acinaciformis, sp. nov. 
(Plate 5, Fig. 58.) 
Type. — Exserted portion of spine; Museum of Comparative Zoology. 
Spines comparatively small, slender, gradually tapering, gently and uni- 
formly arched ; lateral surface with five or six smooth continuous longitudinal 
ridges; posterior denticles slender, rather widely spaced. 
This species is noticed here principally for the sake of comparison with the 
preceding, and to illustrate the difference in degree of curvature pervading 
various spines included under the same genus. Indeed, if we may depend 
upon the determinations of J. W. Davis, spines belonging even to a single 
