150 IOWA GEOLOGICAL SURVEY 



ous species of Harpaeanthus, are in reality the head-spines of 

 Palaeozoic Chimaeroids.* 



ICHTHYODORULITES. 



As remarked by Smith Woodward, the characters of the der- 

 mal spines and tubercles of cartilaginous fishes vary so much in 

 the different genera, and are sometimes so completely identical 

 when other parts are quite distinct, that all fossils of this nature 

 hitherto discovered in an isolated condition may be conveniently 

 grouped together under the denomination of Ichthyodorulites. 

 The term was first employed by Buckland and De la Beche, who 

 were the earliest to discover the true nature of these fossils; 

 it was subsequently applied by Agassiz to all fossil spines of 

 Elasmobranch and Chimaeroid fishes, whether correlated with 

 the teeth or not; and still later was restricted by Smith Wood- 

 ward to those detached spines, tubercles and plates which ex- 

 hibit the microscopical structure of vasodentine, but whose pre- 

 cise systematic position cannot be determined. It is in the 

 latter sense that the term is here employed to include isolated 

 dermal structures that are probably of non-Chimaeroid nature. 



Genus onchis Agassiz. 



Spines of small size, laterally compressed; sides of exserted 

 portion ornamented with smooth or faintly crenulated longitud- 

 inal ridges; no posterior denticles. In all, two Silurian and 

 one Devonian species are known from North American Palae- 

 ozoic rocks, and an undescribed form is also reported from the 

 Niagara of Cumberland, Maryland. The Devonian spine may 

 be briefly indicated as follows. 



Onchus rectus Eastman. 

 (Plate III, Fig. 9) 



1899. Onchus rectus C. R. Eastman, 17th Ann. Rept. N. Y. State Geol. p. 323, 

 text- fig. 4. 



1907. Onchus rectus C. R. Eastman, Mem. N. Y. State Mus. lO, p. 74, text- 

 fig. 16. 



Spines attaining a total length of about 5 cm, nearly recti- 

 linear, and tapering gradually to an acute point. Inserted por- 

 tion round in cross-section, very delicately striated; exserted 



* Carnegie Inst. Wash. Pub. no. 32, 1906, p. 136. 



