10 Mr. A. S. Woodward on the 



II. Fish-Fauna of the Upper Devonian. 

 Subclass ELASMOBRANCHII. 



ICIITHYODORULITES. 

 Genus PSAMMOSTEUS. 



Psavniwsteus orenatus, Agassiz. (PI. II. fig. 11.) 



1845, Psammostem arenatus, L. Agassiz, Poiss. Foss. V. G. 11. p. 105, 



pi. xxxi. figs. 7-10. 

 1884. " Bony fragmeut," E. II. Lanliester, op. cit. p. 6, pi. iv. fig. 17. 



This species has hitherto been met with only in the Devo- 

 nian of North-west Russia and Caithness, and in boulders 

 scattered over the plain of Silesia ; but several typical though 

 fragmentary plates occur in the collection from the ironstone 

 of Mimers Valley, and the writer has been able to verify their 

 reference to an Elasraobranch exoskeleton by the examina- 

 tion of microscopical sections. Some of the plates are very 

 stout, measuring as much as 0*006 in maximum thickness ; 

 but the tissue seems to be everywhere cancellated beneath the 

 external layer. 



Unfortunately none of the specimens completely exhibit 

 their original contour ; but some portions of the free borders 

 are recognizable, and one small slab of ironstone seems to 

 show two pairs of nearly flat plates in natural juxtaposition. 

 From the absence of ornament along an area bordering the 

 free margin in several instances it is obvious that the plates 

 either mutually overlapped or were covered at the edges with 

 integument ; while the slab just mentioned, if rightly inter- 

 preted, indicates that the dermal armature was arranged in a 

 bilaterally symmetrical manner. On the slab in question the 

 inner pair of plates is coarsely ornamented except along the 

 borders of a narrow elongated fontanelle which separates 

 them throughout the greater part of their length mesially; 

 and the remains of the outer pair of plates flanking these 

 indicate that they were much more finely tuberciilated. 

 Some of the abraded stellate tubercles are shown, enlarged 

 about four times, in PI. II. fig. l\h. 



In addition to the broad flattened plates there is one speci- 

 men of much interest, represented in front view and trans- 

 verse section in PI. 11. figs. 11, 11«. It is part of a long- 

 narrow element, bent at its thickened, mesial, longitudinal 

 line, and ornamented by stellate tuberculations, wiiich are 

 ovate rather than round. The modified form of the tubercles 

 is doubtless due to the shape of the plate, wJiich seems to 



