128 IOWA GEOLOGICAL SURVEY 



This species is interesting in the same way as are R. secans 

 and the two species respectively of Ptyctodus (P. calceolus and 

 P. ferox) and Palseomylus (P. crassus and P. greenei), in which 

 the complete dentition is known. The first-named of these 

 species furnishes reliable criteria for distinguishing between 

 upper and lower dental plates, a group of four naturally asso- 

 ciated dental plates being preserved, in which the entire outline 

 of the Meckelian cartilage is shown by the mandibular pair. 

 The lower plate is invariably deeper, and in some forms also 

 somewhat longer than the upper. It usually shows conspicuous 

 symphysial facettes for the lodgment of cartilage holding the 

 jaw-parts together in front. Occasionally, too, it develops a 

 downwardly directed, recurved spiniform process, whose func- 

 tion seems to have been to strengthen the symphysial union, and 

 to form a rigid "chin," so to speak, the front profile of the 

 lower jaw being about as steep as in recent Chimaeroids. The 

 presence of this process can hardly be regarded as a generic 

 character, since it is common to different species of both Ptyc- 

 todus and Rhynchodus, though it has not been observed in any 

 example of Palasomylus. Morphologically the anterior process 

 is equivalent to the distinct chisel-shaped presymphysial tooth 

 of Myriacanthus and Chimaeropsis, from Jurassic rocks. 

 Jaekel's novel interpretation of this structure, and of the sym- 

 physial facettes (which he thinks served to lodge the nasal cap- 

 sules), is due to his confusion of upper and lower dental plates 

 in the species we are considering. We have amended his pro- 

 posed arrangement by restoring the elements to their normal 

 position, as shown in our Plate III, Fig. 8. 



Remains of this species occur typically in the Middle Devonian 

 of the Eifel District, in Rhenish Prussia, and so far as can be 

 determined from published figures and descriptions, also in the 

 Upper Devonian of Wildungen, Waldeck. Jaekel himself fails to 

 note any characters which separate his so-called " Rhamphodus 

 tetrodon" from the type of Baron von Huene's earlier described 

 Rhynchodus emigratus, due regard being paid to the fact that 

 in the figured example of the latter the anterior spiniform proc- 

 ess is missing, and has probably been broken away. The Baron 

 has since stated in correspondence that the only reason which 



