184 IOWA GEOLOGICAL SURVEY 



against one another. The result reached by the present writer 

 after application of this test to a large series of Mylostomid 

 dental plates, all in fact that are anywhere procurable, is that 

 the earlier of the two rival reconstructions of Mylostomid denti- 

 tion is inadequate, since it fails to explain all the observed facts, 

 and is inconsistent with some of them. According to the newer 

 interpretation, the upper dentition of Mylostomids and Dinich- 

 thyids is reducible to a common plan, which is in itself a com- 

 paratively slight modification of that found in typical Dipnoans ; 

 the jaw parts operate in the usual manner, that is to say with- 

 out anomalous movements; and there is no reason to suppose 

 these parts to be non-homologous with the jaws of ordinary 

 fishes.* 



Family COCCOSTEIDAE. 



Cranial shield consisting of few elements, namely, a median 

 and two external occipital plates, in front of which is a single 

 pair of large centrals more or less in contact along the median 

 line; these are in turn preceded by a pair of large preorbitals, 

 which are either wholly or partially separated from each other 

 by the azygous pineal and rostral ("ethmoid") elements; pos- 

 tero-lateral border of the shield formed by marginals and post- 

 orbitals. Orbits not completely enclosed within the shield, 

 bounded inferiorly by a single suborbital plate, behind which 

 occur one or two opercular elements. Upper dentition consist- 

 ing of a pair each of vomerine and palato-pterygoid elements 

 (commonly known, however, as "premaxillaries" and "maxil- 

 laries"), the latter with trenchant functional margin, often 

 serrated or denticulated; lower dental plate intimately fused 

 with the forward portion of the supporting splenial, turned 

 more or less upright with sharp sectorial margin, often serrated 

 or denticulated like the upper. Symphysial margin also some- 

 times denticulated. 



The structure of the typical genus, Coccosteus,is so well known 

 from the luminous researches of Pander, Traquair, Jaekel and 



* For a further exposition of this view consult the following: Eastman, C. R., 

 Structure and relations of Mylostoma. Bull. Mus. Comp. Zool. 1906, 50, no. 1.- — 

 Mylostomid Dentition. Ibid., 1907, 50, no. 7.— Jaekel, 0., Ueber Pholidosteus 

 etc. ' Sitzungsher. Ges. naturf. Freunde, 1907, no. 6, p. 12. 



