202 IOWA GEOLOGICAL SURVEY 



peculiarity, however, distinguishes it from all species of Coccos- 

 teus and Dinichthys, and points to ancestral relations with 

 Titanichthys, as implied by the generic name. We refer to the 

 large size and transverse elongation of the pineal plate, a pecul- 

 iarity that has hitherto been observed in but a single genus of 

 late Devonian Arthrodires. It is on this account that the small 

 Onondaga form in question is regarded as an early forerunner 

 of the group of extremely modified Coccosteans culminating in 

 Titanichthys. 



The • headshield, itself imperfectly preserved, is unaccom- 

 panied by any other bones of the skeleton. Although, as a rule, 

 little dependence is to be placed upon theoretical association of 

 parts, the question may properly be raised whether there are 

 any other Coccostean remains occurring in the same horizon 

 which correspond in general proportions and superficial orna- 

 ment to the headshield under discussion. Attention rests imme- 

 diately upon two forms: the lower dental plate described as 

 Liognathus spatulatus, and the dorsomedian plate known as 

 Coccosteus occidentalis (ante, p. 186). The possibility is not 

 remote that all three of these detached parts which have re- 

 ceived separate names may actually belong to a single species; 

 in default of proof, however, we have no other precedure than 

 to maintain each provisionally as an independent species, or 

 even genus. At the same time it may not be unworthy of com- 

 ment that the peculiar spiniform prolongation observed in Coc- 

 costeus occidentalis reappears in an abbreviated form in the tri- 

 angular termination of the corresponding plate in Titanichthys. 

 Such a coincidence certainly suggests the idea of an inherited 

 characteristic, and confirms us in the belief that prototypes of 

 the most highly specialized Coccosteans are to be sought as 

 early as the Ulsterian stage in this country. 



The accompanying restoration of the headshield (text-fig. 30) 

 is intended to convey a graphic idea of the arrangement of 

 cranial roofing plates and disposition of the sensory canals. The 

 orbital borders are incompletely preserved, and the posterior 

 margin, which has been broken away in the holotype, is re- 

 stored in the figure after analogy with other Coccosteans. The 

 specific title has reference, it is scarcely necessary to add, to 



