DEVONIAN FISHES OF IOWA 101 



same family or even of the same order, were characterized by 

 teeth of similar form. Owing to the resemblance between the 

 Cladodont and Pleuracanthid type of dentition, it is customary 

 to associate the "family" Cladodontidae provisionally under the 

 same ordinal division as that which includes Pleuracanthus, 

 Didymodus, etc. This has been called Ichthyotomi, in allusion 

 to a curious, symmetrical Assuring of the skull as seen in some 

 of the best preserved crania. No indications of Pleuropterygia 

 have yet been found in the Devonian of Iowa or adjoining States. 



Order ICHTHYOTOMI. 



Notochord persistent; neural and haemal arches and inter- 

 calary cartilage present. Pectoral fin archipterygial ; pelvic fins 

 with claspers, caudal fin diphycercal. No placoid scales, but the 

 head is covered with dermal ossifications. 



This order is represented in the Devonian only by detached 

 teeth similar to those characterizing well-known Pleuracanthid 

 genera, and consisting of two or more sharp cusps, with or with- 

 out intermediate denticles, attached to broad bases. While more 

 specialzed than the preceding order, the fishes included in this 

 group represent an extremely generalized type of Elasmo- 

 branch. Indeed, it has been justly said of Pleuracanthus, the 

 typical genus, that "it is a form of fish which might with very 

 little modification become either a Selachian, Dipnoan, or Cross- 

 opterygian" (Woodward). The general aspect of this creature 

 has become familiar through the frequently copied restoration of 

 Brongniart and Fritsch. During recent years a novel attempt at 

 reconstructing its essential features has been made by Professor 

 Otto Jaekel,* of Greifswald, whose illustration is reproduced 

 herewith (text-fig. 16). It is intended to show an amphistylic 

 skull, five branchial arches bearing clusters of minute denticles, 

 no circumorbital plates, and continuous median fins. 



* Jaekel, O., Neue Rekonstruktionen von Pleuracanthus sessilis etc. Sitzungsber. 

 Ges. Naturforsch. Freunde, 1906, no. 6, pp. 155-159. Some comments on this 

 figure, and also on Jaekel's restoration of Hybodus, are offered by E. Koken in 

 his memoir "Ueber Hybodus", in Geol. u. Pal. Abhandl., 1907, n. f. 5, Heft 4. 



