218 BULLETIN: MUSEUM OF COMPAHATIVE ZOOLOGY. 



Homacanthus delicatulus, sp. nov. 



(Plate 3, Fig. 28; Plate 5, Fig. .59) 



Tyj)e. — 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 

 3 mm. 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. I., 

 1883, PI. XLVIII., Fig. 8.) 



Forniation 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 



