DEVONIAN FISHES OF IOWA 235 



that any other variations of smooth Dipterine dental plates have 

 been discovered. Moreover, the only known occurrence of the 

 Synthetodus type is at two limited localities, the State Quarry 

 and Sweetland Creek "fish-beds", near what was formerly the 

 south-eastern terminus of the Dakotan Sea (cf. Plates XV, 

 XVI, after Schuchert). 



Formation and locality. State Quarry and Sweetland Creek 

 beds (Upper Devonian) ; Johnson and Muscatine counties, Iowa. 



Genus SCAUMENACIA Traquair. 



Body laterally compressed, covered with very thin scales of 

 moderate size. Dermal cranial roof-plates fewer than in Dip- 

 terus, those in the occipital region traversed by an Arthrodiran- 

 like sensory canal system. Anterior margin of headshield even- 

 ly rounded, possibly terminating in front in Ganorhynchus-like 

 labial cartilage. Dentition "ctenodont", the dental plates 

 marked by ridges of well-separated conical tubercles. Paired 

 fins acutely lobate; anterior dorsal arising far forwards, and 

 separated from the long posterior dorsal by a distinct interval. 

 Caudal fin heterocercal ; anal small, separate. 



This genus, represented by the solitary species, S. curta 

 (Whiteaves), and known only from a single locality in the Upper 

 Devonian of Canada, is especially interesting on account of the 

 information it affords in regard to cranial and body characters.* 

 The median series of cranial roof-plates bears comparison with 

 primitive Arthrodires. The lateral series are less *nuinerous 

 than in Dipterus, those in the orbital region are symmetrically 

 arranged and correspond collectively to not more than two 

 plates in typical Arthrodires. In text-figures 34 and 35 are pre- 

 sented for comparison illustrations showing the arrangement of 

 cranial roof-plates in Dipterus and Scaumenacia respectively. 

 Their not too remote correspondence with the patterns shown in 

 text-figures 28 and 29 (ante, p. 197) is not only suggestive, but 

 regarded as very significant from the point of view of interre- 

 lationships. 



*It should be noted, however, that very similar cranial characters are presented 

 by the Australian form described as Ganorhynchus suessmilchi Etheridge, which is 

 undoubtedly very closely related to Scaumenacia, if indeed not identical with it. 

 See the foregoing description of Dipterus murchisoni (p. 226). 



