216 IOWA GEOLOGICAL SURVEY 



vonian Dipterines in the eastern and western provinces respect- 

 ively. A critical examination of the accompanying species in 

 the latter area would no doubt be profitable, for the reason that 

 they all present a multitudinous range of variation, and in so 

 far defy a precise analysis of characters. It was formerly the 

 opinion of the present writer, and in this Professor Calvin and 

 others who have examined the material concurred, that at least 

 several genera and numerous species of Ctenodipterines were 

 represented in the State Quarry deposits of Johnson county, 

 where a most singular assemblage occurs.* That opinion now 

 requires to be modified in the light of experience gained through 

 a study of the varied expressions of the D. nelsoni-fabelliformis 

 type in the eastern area, and the counter-series exemplified by 

 D. mordax in the State Quarry beds. It is evident that a very 

 wide latitude must be assigned to the possible limits of variation 

 and deformation and mutilation produced by well ascertained 

 causes. Besides those aforementioned, there remains to be con- 

 sidered one other, which we have reserved for the conclusion 

 of these general remarks. One perceives, therefore, that the con- 

 sequences of this synthetic mode of treatment is to merge con- 

 ventional distinctions as far as possible, and reduce the number 

 of provisional genera and species to basic terms. In the follow- 

 ing systematic descriptions the closely intergrading series of 

 State Quarry Dipterine teeth are grouped into four specific 

 categories, those of the smooth Synthetodus type into two only. 

 The great mass of worn, effete, imperfect or seemingly abnormal 

 teeth which differ in minor details from the composite or ave- 

 rage expression of the standard species is to be regarded merely 

 as the debris of the latter, and interesting chiefly because of the 

 eloquent commentary it offers as to physical, chemical and per- 

 haps even organic destructive agencies. 



A word is necessary to explain the meaning of this last state- 

 ment. Briefly, the appearance of a considerable number of State 

 Quarry Dipterine teeth suggests a mildly corrosive or solvent 

 action that is difficult to explain by the ordinary process of min- 

 eral replacement, but finds a ready interpretation by assuming 



* First described by Professor S. Calvin in Proc. Iowa Acad. Sci. 1896, 4, pp. 

 16-21. Also in Rept. Iowa Geol. Surv. 1896, VII, pp. 72-79, and 108-116. 



