78 NEW YORK STATE MUSEUM 



None of these spines appear to have exceeded lo cm in length, and 

 they are frequently much shorter, their form being narrow and gradually 

 tapering. The flattened sides are covered with numerous threadlike 

 costae, as many as twenty being observed toward the base, and these are 

 for the most part continuous ; when new longitudinal ridges are formed, 

 they arise by implantation. The costae are finely pectinated at intervals 

 varying from twice to three times their own width, thus giving rise to a 

 finely punctate appearance when seen in impression, as is usually the case. 



Formation and locality. Chemung group ; New York and Pennsylvania. 



Genus acantholepis Newberry 



Spines of large size, very much laterally compressed, thin walled, 

 internally hollow, in form gently arcuate, wide at the base, and gradually 

 tapering to an acute point. Sides of exserted portion nearly flat, orna- 

 mented with numerous fine stellate tubercles ; these are either irregularly 

 arranged, or in some cases exhibit a tendency toward concentric arrange- 

 ment along lines of growth. Tubercles along either side of the posterior 

 margin enlarged so as to form a double row of denticles directed at right 

 angles to the margin. Appearances of at least one specimen suggest that 

 the basal portion is segmented. 



The spines which are provisionally included under this title' were first 

 confused with Oracanthus by Newberry, but subsequently interpreted by 

 him as dermal plates or scutes of " Placoderms," and supposed to be of 

 similar nature to those which have received the name of Acanthaspis. Still 

 later, their resemblance to the triangular ichthyodorulites of Psammosteus 

 was pointed out by Smith Woodward. The conclusion reached by the 

 present writer is that they are dermal defenses of Chimaeroids, probably 

 dorsal fin spines. Their extremely thin walls, as seen under low magnify- 

 ing power in worn specimens, exhibit the structure of vasodentine, and are 

 traversed by coarse, branching longitudinal canals. In external characters 



' For this designation, preoccupied amongst insects, S. A. Miller has proposed the 

 not altogether pleasing sobriquet of " Eczematolepis," an example of ''index-learning that 

 turns no student pale." 



