240 IOWA GEOLOGICAL SURVEY 



Greenland and Spitzbergen, for our knowledge of which we are 

 indebted to Smith Woodward.* 



Family ONYCHodointidae. 



Scales cycloidal, deeply overlapping. Head and opercular 

 apparatus with well developed membrane bones. Dentary bone 

 of mandible thin and deep, bearing a single close series of large 

 conical teeth, flanked by an outer series of very minute teeth ; an 

 azygous series of large, more or less recurved teeth attached in 

 front of the symphysis. Teeth plicated only at the base, with 

 a central cavity ; dentary teeth tipped only, presymphysial teeth 

 completely enveloped with enamel. 



This family is at present known to be represented in the De- 

 vonian of Europe and North America by but a solitary genus, 

 Onychodus, whose remains invariably occur in a fragmentary 

 condition. The various bones of the cranial roof and pectoral 

 girdle that have been found suggest a certain resemblance to 

 Holoptychians and Ehizodonts. The tooth structure, however, is 

 simple, and a conspicuous difference exists in the presence of 

 a dentigerous presymphysial bone. The external bones and 

 scales of the type species, 0. sigmoides Newberry, are orna- 

 mented with fine tuberculations, more or less conical and radially 

 grooved. The clavicle is triangular in shape, with relatively 

 large inferior limb; the infraclavicle is without an elongated 

 ascending process. In this species, also, the presymphysial 

 bone is very prominent, its teeth being much larger than those 

 of the dentary. Remains of 0. sigmoides are not uncommon in 

 the Columbus and Delaware limestones of Ohio, and probably 

 occur also in the Onondaga limestone of LeRoy, New York. A 

 few detached presymphysial teeth not readily distinguishable 

 from this form are also known from the Hamilton of Milwau- 

 kee, Wisconsin, an imperfect one of this nature being shown in 

 Plate I, fig. 4; others, with less pronounced curvature and ellip- 

 tical cross-section are present in the Middle Devonian of the 

 Eifel District. As an example of the latter, a single tooth be- 



* Woodward, A. S., Notes on fossil fish-remains collected in Spitzbergen by the 

 Swedish Arctic Expedition, 1898. Bihang till Svenska Vet.-Akad. Handl. 1900, 

 vol. 25, no. 5, pp. 1-7. — Idem, Notes on some Upper Devonian Fish-remains dis- 

 covered by Prof. Nathorst in East Greenland. Ibid. 1900, vol. 26, no. 10, 

 pp. 1-10. 



