DEVONIAN FISHES OF IOWA 119 



oral surface is triturated by wear. In the surviving family of 

 Chimaeridae these dental plates are much thickened, while the 

 hinder upper pair ("palatines") are both closely apposed in 

 the median line and considerably extended backwards. 



The dental plates named Ptyctodus, from the Devonian of Rus- 

 sia, Rhenish Prussia and North America, are essentially simi- 

 lar to those of modern Chimaeroids, but there are no represen- 

 tatives of the vomerine pair. The tritors, one only in each plate, 

 are well differentiated, consisting of hard, punctate superim- 

 posed laminae, arranged obliquely to the functional surface. The 

 contemporaneous teeth known as Rhynchodus and Palaeomylus, 

 however, exhibit more indefinite tritoral areas, or none. The 

 symphysial facette is always distinct. 



Spines which may be compared with those of modern Chimae- 

 roids are also known from the Devonian and Carboniferous sys- 

 tems, and Harpacanthus and Cyrtacanthus may perhaps be cited 

 as examples of head spines. No Chimaeroid skeletons, however, 

 have hitherto been satisfactorily determined from Palaeozoic 

 rocks, save for the possible exception of Menaspis.* The frag- 

 mentary remains known as Dictyorhabdis priscus, from the Or- 

 dovician of Canyon City, Colorado, are of extremely problem- 

 atical nature, and it is even questioned by some authors whether 

 they are vertebrates. 



The most recent and thoroughgoing discussion of the rela- 

 tions between fossil and recent Chimaeroids is that of Professor 

 Bashford Dean, in publication No. 32 of the Carnegie Institu- 

 tion of Washington, 1906. His more general conclusions are 

 summed up in the following paragraph (p. 155) : 



Chimaeroids, accordingly, are widely modified rather than 

 primitive forms. The evidence contributed by anatomy, embryol- 

 ogy and palaeontology is unmistakably in favor of this interpre- 

 tation. And there can be no doubt that the recent forms retain 

 less perfectly the general characters of the ancestral gnatho- 

 stome than do living sharks. On the other hand, it must be ad- 

 mitted that Chimaeroids have retained several characters of 

 their Palaeozoic Selachian ancestors which modern sharks have 

 lost. According to many converging lines of evidence we may 



* Skeletal portions in natural association with the dental plates of the type spec- 

 imen of "Rhamphodus" are reported, but have not yet been described, by Dr. 

 Otto Jaekel. 



