134 IOWA GEOLOGICAL SURVEY 



1899. Ptyctodus calceolus S. Weller, Journ. Geol. 7, p. 484. 

 1906. Ptyctodus sp. B. Dean, Carnegie Inst. Wash. Pub. no. 32, p. 137, 

 text-fig. 116. 



1906. Ptyctodus calceolus W. H. Norton, Rept. Geol. Surv. Iowa (1905), 



XVI, p. 356. 



1907. Ptyctodus calceolus G . R. Eastman, Mem. N. Y. State Mus. 10, p. 71. 



Dental plates compressed into a thin cutting edge shortly be- 

 hind the symphysis, but widening gradually, becoming more or 

 less outwardly curved, and the functional surface occupied for 

 nearly its entire width by the tritoral area, the inner margin of 

 which is more strongly curved than the external. Laminar struc- 

 ture of the tritors indicated superficially by fine punctae arranged 

 in parallel rows which are directed obliquely across the triturat- 

 ing surface. The compressed edge in advance of the tritor in 

 the lower dental plate slopes rapidly upward and terminates in 

 a strong anterior beak, beneath Which the front margin is con- 

 tinued downwards in a short, blunt process. Upper dental 

 plates similar in a general way to the lower, except that the sym- 

 physial margin is rounded and not produced into a beak. 



Notwithstanding the extraordinary abundance of this species 

 in various Middle and Upper Devonian localities of the Missis- 

 sippi Valley, as indicated by thousands of detached tritors, com- 

 plete dental plates are very rare, their structure being on the 

 whole frail and liable to injury. Of widespread distribution in 

 the Hamilton, the species is most profuse in limited areas of 

 the Upper Devonian, certain layers of no great thickness being 

 fairly charged with their broken and abraded tritors. Nowhere 

 have similar remains been found in such abundance as in the 

 old State Quarry beds * near North Liberty, in Johnson county, 

 where a single exposure only a few yards square has yielded 

 countless fragments of the dentition, including a score or more 

 nearly perfect plates. They are plentiful, though generally of 

 smaller size, in the contemporaneous Sweetland Creek beds of 

 Muscatine county; fine specimens have been obtained from 

 the Upper Devonian Lime Creek shales of Cerro Gordo county, 

 and perhaps the most perfect of all from the Cedar Valley lime- 

 stone of Bremer county, Iowa. Well preserved dental plates of 



*Calvin, S., Geology of Johnson county. Rept. Geol. Survey Iowa, 1897, VII, 

 pp. 74-76. Also Proc. Iowa Acad. Sci. 1897. 4, pp. 16-21. 



