DEVONIAN FISHES OF IOWA 167 



position of Bothriolepis, which is summed up as follows: "The 

 structure of the gills, anus, anal fin and other organs indicate 

 that the Ostracoderms must be separated from all other known 

 subdivisions of the Chordata and raised to the dignity of a sep- 

 arate class." * 



Family MACROPETALICHTHYIDAE. 



Cranial shield much arched from side to side, completely en- 

 closing the orbits, and extending over the nuchal region poster- 

 iorly. External surface covered with fine stellate tubercles which 

 conceal the underlying sutures between dermal plates. Median 

 series of the latter but two in number, narrow and elongate; 

 external occipitals large; centrals divided, the two pairs on 

 either side not meeting their fellows in the median line. Pineal 

 foramen inconspicuous, situated slightly in advance of a line 

 joining the anterior borders of the orbits. Sensory canals form- 

 ing large tubular excavations in the bone, opening at the ex- 

 ternal surface by a continuous narrow slit or by a double series 

 of pores. Parachordal cartilage and notochordal sheath calci- 

 fied. Nature of dentation unknown, although there is apparently 

 an articular cavity for the lower jaw. No indications of ab- 

 dominal armoring. 



The typical genus of this family is Macropetalichthys, known 

 by a single American, and two or three European species. In 

 addition-, an undescribed species is reported by Jaekel from the 

 Middle Devonian of the Eif el District, in Rhenish Prussia, the 

 type of which is in the Senckenburg Museum at Frankfort, and 

 at the same time the form described by Kayser in 1880 as M. 

 pruemensis is made by Jaekel the type of a distinct genus. f All 

 these forms evidently stand in close relation to Homosteus as 

 regards number and general arrangement of cranial roofing 

 plates, position of the orbits, and in having the headshield pro- 

 longed posteriorly over the nuchal region. It is perhaps of 

 some significance to note that the median series of cranial plates 

 are reduced to the same number as in Neoceratodus, and are 

 fewer than in Dipterus. 



* Science, 1905, 21, p. 297.— See also Yearbook Carnegie Inst. Wash. 1904, no. 

 3, p. 140, where the same conclusion is presented. 



t'Jaekel, 0., Ueber Coccosteus und die Beurtheilung der Placodermen. Sit- 

 zungsber. Ges. Naturforsch. Freunde, 1902, p. 113. — Ibid., 1906, pp. 73-85. 



