74 IOWA GEOLOGICAL SURVEY 



Orders ANASPIDA and OSTEOSTRACI. 



Neither of these orders is represented in the fossiliferous 

 rocks of the United States. A single species described as 

 Euphanerops longaevus by Smith Woodward is known from the 

 Upper Devonian of Scaumenac Bay, Province of Quebec, Can- 

 ada, and the Osteostraci are represented by four species of Ceph- 

 alaspis (text-fig. 4, page 58), two from the Lower, and two from 

 the Upper Devonian of British North America. Notwithstand- 

 ing their rarity in rocks of the western hemisphere, these forms 

 enjoyed a cosmopolitan distribution during the Devonian, as is 

 shown by the recent discovery of Thyestes in Australia.* 



It has been claimed by Professor Patten that the genus Ceph- 

 alaspis is provided with "a fringe of jointed and movable ap- 

 pendages (25 to 30 pairs) along the ventral margin of the 

 trunk," the structures commonly known as marginal scales 

 being interpreted by him as swimming-organs, or as he calls 

 them, "fringing processes". These fulcra -like scales were even 

 regarded by this author at one time as probable "antecedents 

 of the lateral fold of vertebrates"; but in the light of Dr. Gas- 

 kell's re-examination of their structure, and the unanimity of 

 opinion in the minds of all other students that paired append- 

 ages do not occur, we may continue to believe that the term 

 marginal scales is not a misnomer for the structures in question.! 



Order ANTIARCHI. 



The Antiarchi of the Devonian possess a much more complex 

 system of dermal plates than other groups of Ostracophores, 

 and are provided with a pair of singularly jointed armored ap- 

 pendages, usually movable, evidently serving as organs of pro- 

 gression, and totally unlike the limbs of other vertebrates. t The 

 head- and body-shields are always movably articulated, and both 

 are traversed by well defined sensory canals. The bone structure 

 is dense, though with vascular cancellae in the middle layer of 



* Cf. F. Chapman, in Proc. R. Soc. Vict., 1906, n. s., 18, pp. 93-100. 



tCf. W. H. Gaakell, in Journ. Anat. and Physiol., 1903, 37, p. 198. Also 

 Dr. O. Jaekel in Zeitschr. deutsch. geol. Ges. 1903, 55, p. 84, and articles by the 

 present writer in Amer. Nat. and Science for 1903 and 1904. 



J For references to different views of the nature of these organs, see an article 

 on Asterolepid appendages in Amer. Journ. Sci., 1904, ser. 5, 18, pp. 141-144. 



