86 NEW YORK STATE MUSEUM 



is the case, instead of there being merely a single, large broken spine, is 

 evident from the similar proportions and general appearance of the two 

 spines, one of which clearly represents the proximal and middle portions, 

 and the other a section extending from about the middle for a considerable 

 distance beyond in the direction of the e.pex. The form of the cross-section 

 leaves no doubt that both spines present the same aspect, presumably the 

 outer or external face. On the opposite, or internal face, the median 

 carina appears to be gently rounded throughout its entire length. One of 

 the distinguishing characteristics of this species, however, is that the axial 

 ridge on the side which is presumed to be external remains sharply triangu- 

 lar only in the distal half of the spine, becoming widened into a broad flat 

 elevation, smooth or but faintly striated, and nearly rectangular in cross- 

 section, toward the base of the spine. The general surface is smooth, save 

 for the usual delicate striae, slightly convergent toward the apex, and pos- 

 sibly of the same nature as growth lines. The absence of deep longitudinal 

 grooves distinguishes this species from M. sulcatus and M. b o h e m i- 

 c u s, the latter having been originally referred to Ctenacanthus. A 

 few doubtful fragments, suggestive either of this species or the preceding, 

 have been obtained from the Hamilton of Milwaukee, Wis. 



Formation and locality. Hamilton limestone (Erian) ; Eighteen Mile 

 creek, N. Y. (and Wisconsin ?). 



Genus gykacanthus Agassiz 

 The paired fin spines of this genus,' which are not known to be associ- 

 ated with other types of dermal defenses in the rocks of this country, are 

 thus described by Smith Woodward : 



Spines of two distinct types, the one evidently connected with the fins, 

 the other free. Fin spines elongated, robust, more or less arched, irregu- 

 larly rounded or oval in transverse section, except towards the unworn 

 apex, which is compressed ; base of insertion large, with the internal cavity 



' AVoodward, A. S. On a Carboniferous Fish Fauna from the, Mansfield District, Vic- 

 toria. Nat. Mas. Melbourne. Mem i, 1906. p. 1-32. In this paper the relations of the 

 " Gyracanthidae " are definitely proved to be with Acanthodian sharks, upon the evidence 

 of a complete skeleton of the remarkable Australian genus Gyracanthides [text fig. 17]. 



