130 IOWA GEOLOGICAL SURVEY 



more remarkably suggestive of the spiniferous plates described 

 as Acanthaspis armata and A. pruemensis. The last-named 

 form, it should be remarked parenthetically, is found in the 

 same horizon with Rhynchodus dental plates in the Lower De- 

 vonian of the Eifel District, and the former accompanies the 

 type species of Rhynchodus in the Middle Devonian of Ohio. 



That these plates were externally situated, and therefore not 

 to be interpreted as parts of a primary pectoral girdle (such 

 as occurs in ganoids and teleosts), is patent from their extreme 

 tenuity, fragility, tubercular ornamentation, and the dense, brit- 

 tle and close-grained texture of their superficial layer, which 

 has the glistening appearance of vasodentine. Were they the 

 parts of an osseous secondary girdle, such as we are acquainted 

 with in dipnoans, ganoids* and teleosts, we must suppose them 

 to be membrane bones developed on the outer side of the primary 

 girdle and re-enforcing it. But as shown by examination of a 

 large series of detached plates presumably belonging to R. 

 excavatus, there is absolutely no indication that they were at- 

 tached on their inner or visceral side to any other structures, 

 as they must needs have been according to this idea of their 

 nature. On the contrary, all appearances indicate that the series 

 of scale-like plates which Jaekel identifies as a secondary bony 

 shoulder-girdle in "Rhamphodus" were merely dermal ossifica- 

 tions, loosely supported in the integument, and forming an os- 

 seous chain or band arranged transversely in advance of the pec- 

 toral fin and back of the head-region, but having nothing what- 

 ever in common with the cartilaginous pectoral arch, and in 

 nowise homologous with the lateral body-armoring of Arthro- 

 dires. Evidence is wanting which justifies an association of 

 Ptyctodonts with Arthrodires, and the known evolutionary his- 

 tory of Chondrostei will not permit us to affiliate them with 

 modern Sturgeons. 



On the assumption that Ptyctodonts are of Arthrodiran na- 

 ture we should expect to find wherever their remains are favor- 

 ably preserved, (1) above all things an ossified headshield; (2) 

 two pairs of dental elements in the upper jaw, one of them cor- 



*For a discussion of the secondary shoulder-girdle in Chondrostei see R. Wied- 

 ersheim's "Das Gliedmassenskelet der Wirbelthiere," 1892, pp. 155-160. Con- 

 sult also the literary references given in this and other works by the same author. 



