76 IOWA GEOLOGICAL SURVEY 



plate. Position of the nasal organs not definitely known. One 

 pair of paddle-like swimming appendages, completely encased 

 in osseous plates, and articulated by a complex joint with the 

 anterior ventro-lateral plates of the trunk. Sensory canal sys- 

 tem well developed. Tail either naked or scaly ; one or two dorsal 

 fins, and a completely heterocercal tail in at least the typical 

 genus. 



To form an adequate conception of the creatures indicated by 

 numerous detached fragments of Asterolepid armor from the 

 Upper Devonian of the eastern United States and Colorado, it 

 is necessary to pay strict attention to recent work that has been 

 done in investigating Scottish and Canadian species. The prin- 

 cipal advance that has been made during late years is owing to 

 the researches of Dr. E. H. Traquair, of Edinburgh, Professor 

 William Patten, of Dartmouth, and Professor Otto Jaekel, of 

 Greifswald, Germany, whose results are scattered through a very 

 considerable number of papers. Without doubt the most not- 

 able contribution, and at the same time one of the most author- 

 itative, is Dr. Traquair 's Monograph on the Aster olepidae, still 

 in course of publication by the Palasontographical Society of 

 Great Britain. As students who have not access to special' li- 

 braries can hardly be supposed to be familiar with the parts* 

 of this work already published, we shall probably do well to 

 present here a revised account of the Asterolepid organization, 

 which has been very slightly condensed from the Scottish 

 author's descriptions. This account follows immediately. 



Structure of Asterolepids, as illustrated by Pterichthys. — Be- 

 fore entering into a description of individual forms, it is first of 

 all necessary to understand the general structure of an Aster- 

 olepid, and to know the names by which the various parts found 

 in the fossil state may be distinguished. For this purpose it is 

 well to take Pterichthys (Figs. 9-11), as being the genus best 

 known in its entirety, if not in every manner of detail. 



The body armor consists of osseous plates closely fitted to- 

 gether, closed above, below and at the sides, but open in front 

 for the head, and behind for the tail. The head is covered almost 

 entirely by a dorsal shield, formed also of plates united by su- 



*Part ii. No. 1, 1894; No. 2. 1904; No. 3, 1906. 



