188 IOWA GEOLOGICAL SURVEY 



and 1895, and by Giirich in 1891 (Zeitsehr. deutsch. geol. Ges. 

 43, p. 907), differs to an extent that readily permits generic sep- 

 aration, and the spinous appendage in the pectoral region is 

 always much more strongly developed than in the single species 

 of Coccosteus where it is known to occur. In the species de- 

 scribed by von Koenen as B. bidorsatus, and subsequently re- 

 named Pholidosteus friedelii by Jaekel, the spiniform process 

 is represented as being fused with the lower border of the antero- 

 lateral plate, thus recalling the very similar conditions observed 

 between one of the plates of the ventral armor and its lateral 

 spine in Phylctaenaspis acadica (Whiteaves). Analogous con- 

 ditions exist also in the singular Ostracophore described by 

 Smith Woodward from the Lower Devonian of Spitsbergen 

 under the name of Acanthaspis decipiens* which we have pro- 

 posed to make the type of an independent genus. It is evident 

 that the spinous process of this Spitzbergen form is non-homo- 

 logous with the segmented spiniform projection of those plates 

 now known to belong to Ehynchodus, such as were formerly de- 

 scribed under the name of Acanthaspis armata, nor can it by 

 any possibility be homologized with the so-called "pectoral 

 spines" of Coccosteus and Brachydirus. 



Genus DINICHTHYS Newberry. 



It is difficult to frame a satisfactory diagnosis of this genus 

 which shall enumerate its principal characters and at the same 

 time enable one to draw a rigid distinction between its various 

 species and those of Coccosteus. The fact is, the two genera 

 are most intimately related, and though their terminal members 

 are sufficiently well characterized, they are connected by insen- 

 sible gradations. The typical species of Dinichthys represent 

 unquestionably a later and more advanced stage of specializa- 

 tion than that with which we are familiar in Coccosteus de- 

 cipiens, for example; but between these extremes lies a host 

 of intermediate forms. Evidence of specialization in forms 

 like D. herzeri, D. terrelli, etc., is strikingly apparent in their 

 gigantic size, the headshield measuring nearly a meter across, 

 and their massive and cumbersome armor being unrivaled among 



* Ann. Mag. Nat. Hist. 1891 , ser. 6, 8, p. 4. See also Dr. Traquair's observations 

 in the same magazine for 1894, 14, p. 371. 



