DEVONIAN FISHES OF IOWA 



55 



derived from the recent studies of Dr. Otto Jaekel, in Germany, 

 and Professor William Patten, of Dartmouth College, in this 

 country. 



Without entering at this point into the basis of classification, 

 it may be remarked in passing that the different families of Os- 

 tracophores are commonly arranged in four ordinal divisions, of 

 which the Heterostraci are regarded as the most primitive. This 

 order comprises the curious group of Pteraspidians, the struc- 

 ture of whose hard shield is unique among vertebrates, and also 

 the remarkable forms included within the families of Coelolepida? 

 and Psammosteida?. Especially noteworthy among Coelolepids 

 are the genera Thelodus (Fig. 1) and Lanarkia (Fig. 2), known 



Fig. 1. 



Fig. 2 



Fig. 1. Thelodus scoticus Traq. Silurian (Ludlow beds) of Logan Water. Diagrammatic 

 restoration showing dorsal fln and position of the eyes. The tail is flexed so as to show the 

 caudal fln in profile, x I (after Traquair) . 



Fig. 2. Lanarkia spinosa Traq. Silurian (Downtonian) , of Scotland. Diagrammatic 

 restoration similar to that of Fig. 1. Dorsal fln not yet observed, x I (after Traquair). 



