146 IOWA GEOLOGICAL SURVEY 



1896. Acanthaspis armata E. W. Claypole,- Amer. Geol. 17, p. 354. 



1906. Acanthaspis sp. L. Hussakof, Mem. Amer. Mus. Nat. Hist. 9, p. 132, 



text-fig. 16. 

 L907. Acanthaspis armata C. R. Eastman, Mem. N. Y. State M us - 10, p. 117, 

 pi. 2, fig. 2. 



1907. Acanthaspis armata O. Jaekel, Sitzber. Ges. naturf. Freunde, no. 6, p. 



8, text-fig. 4. 



The spinif erous plates which are known under this provisional 

 designation have not been found in natural assemblage with 

 other parts of the skeleton of forms accompanying them in the 

 same horizon, but it is to be noted that they are an invariable 

 accompaniment of Rhynchodus dental plates, and as essentially 

 similar structures have recently been described by Jaekel in 

 undoubted association with the dentition, calcified cranial roof 

 and other parts of the anatomy of Rhynchodus major (type of 

 his so-called "Rhamphodus"), the most satisfactory interpreta- 

 tion of their nature seems to be to regard them as dermal ossi- 

 fications of the same genus. 



All of the specimens thus far brought to light are of fairly 

 uniform configuration, and agree in being composed of two por- 

 tions: (1) a thin, almost lamellar and more or less flattened 

 plate, finely tuberculated on the external surface in larger ex- 

 amples, but smooth or with but few scattered tubercles in those 

 of smaller dimensions; and (2) an elongate, tapering, gently 

 curved and feebly denticulate spinous process, which is immov- 

 ably attached to the supporting lamellar plate by an oblique 

 suture. This suture may be indistinct in small-sized examples, 

 but is usually conspicuous in those whose larger size may be con- 

 sidered as indicating adult individuals. The interior of the spin- 

 ous portion is hollow throughout, subcircular in cross-section, 

 although frequently deformed by the fossil ization process, and 

 along the line of its attachment with the basal plate is some- 

 times to be seen a double row of perforations which communi- 

 cate with the internal cavity. 



Formation and locality. Onondaga limestone; Le Roy, New 

 York, and the nearly equivalent Columbus and Delaware lime- 

 stones of Ohio. 



