DEVONIAN FISHES OF IOWA 123 



support of the contention that Ptyctodonts are Arthrodires, and 

 that modern Chimaeroids are specialized descendants of Cochlio- 

 dont sharks. The capital point which influenced him in favor of 

 the former of these theses, as the author admits, depended upon 

 Jaekel 's unconfirmed statement that a bony shoulder-girdle 

 was present in "Khamphodus" the elements of which were 

 thought to be disposed somewhat after the pattern of Coccos- 

 teid dermal plates. Evidence will be presented hereinafter to 

 show that both Jaekel and Dollo are mistaken in their inter- 

 pretation of the elements forming a supposed pectoral arch, and 

 that real homologies do not exist between these external ossi- 

 fications and parts of the Arthrodiran system of body-armoring. 



Genus RHYNCHODIS Newberry. 

 (Syn. Bhamphodus Jaekel non Davis.) 



Functional margin of dental plates simply trenchant, without 

 tritoral areas. Upper and lower dental plates of similar form, 

 except that in some species the symphysial margin of the lower 

 plates is produced downward into a spiniform process. Indi- 

 cations of cartilaginious union between each pair of dental plates 

 are distinctly shown on the inner symphysial facettes. 



Bhynchodus secans Newberry. 



1873. Bhynchodus secans J. S. Newberry, Eept. Geol. Surv. Ohio, 1, pt. 2, 



p. 310, pi. 28, fig. 1; pi. 29, figs. 1, 2. 

 1889. Bhynchodus secans J. S. Newberry, Monogr. U. S. Geol. Surv. 16, 



p. 17, pi. 28, figs. 1-3. 

 1898. Bhynchodus secans C. E. Eastman, Amer. Nat. 32, pp. 485, 546. 



1906. Bhynchodus secans B. Dean, Carnegie Inst. Wash. Pub. no. 32, p. 139, 



text-fig. 127. 



1907. Bhynchodus secans C . E. Eastman, Mem. N. Y. State Mus. 10, p. 67. 



This species, which is the type of the genus, is not uncommon 

 in the Columbus and Delaware limestones of Ohio, and is inter- 

 esting for having furnished a group of four teeth preserved in 

 natural association, evidently representing the complete den- 

 tition of a single individual. In this the mandibular dental plates 

 are easily recognized as such by reason of their having retained 

 an impression showing the entire outline of the Meckelian carti- 

 lage. Aside from this, there is but little difference in the form of 



