170 IOWA GEOLOGICAL SURVEY 



fail of comprehension, or at least of satisfactory analysis, save 

 as they are brought into relation with those of modern Dip- 

 noans and interpreted through comparison with them. Many 

 students have puzzled over the cranial structure of Macropet- 

 alichthys and the allied genus Asterosteus, of which only the 

 median series of plates are known; but accumulation of details 

 has resulted only in greater perplexity. A serious obstacle to 

 their understanding has been the absence of a standard of com- 

 parison or trustworthy clue by means of which their characters 

 acquire significance ; they must needs remain unintelligible until 

 brought into harmonious adjustment with other established 

 facts. 



Newberry, with an abundance of well preserved material at 

 his command, went widely astray in imagining these forms to 

 be ancestral to modern sturgeons. Cope's keen insight led him 

 immediately to perceive the community of structural plan be- 

 tween Macropetalichthys and Dinichthys; and in suggesting a 

 comparison of the former with Neoceratodus, he actually hit 

 upon a solution of the whole matter, though unfortunately he 

 did not rigorously apply it. He correctly identified the paras- 

 phenoid as such — a membrane bone that appears to have been 

 incompletely formed in Arthrodires generally — and noted that 

 it displayed the usual Dipnoan outline ; but he was less happy in 

 explaining the nature of the so-called "cerebral chamber" of 

 Newberry, and other internal structures termed by him "nuchal 

 elements". 



Cope's "nuchal plate", or so-called "dorsal plate" of Dean 

 and Eastman, was further misinterpreted by the last-named 

 authors in that it was held to represent collectively the dorsal 

 body plates of other Arthrodires. Dean's definition of "An- 

 arthrodira" was, in fact, based upon this erroneous view.* 

 Indeed, it must be frankly acknowledged that serious misap- 

 prehension has prevailed among all students, including the pres- 

 ent writer, concerning the septate structures within the interior 

 of the. headshield of Macropetalichthys. As in the case of the 

 cranial buckler itself, the conformation of the inner parts be- 



* As pointed out by 0. Jaekel in the Neucs Jahrbuch for 1903 (1, p. 342), the 

 definition embraces structural characters which in reality do not exist. 



