DEVONIAN FISHES OF IOWA 207 



genus and the imperfectly known Glyptaspis. Phyllolepis is 

 distinguished from Holonema by having the superficial rugae 

 arranged in concentric, instead of radiating lines. The only 

 American species that has been described is P. delicatula New- 

 berry, from the Chemung of Bradford county, Pennsylvania. 

 Accompanying the latter, but of more dubious nature, are plates 

 with arrowhead-like ornamentation, described by Newberry un- 

 der the name of Sphenophorus lilleyi* Certain elliptical plates 

 having a closely similar style of ornamentation are also known 

 from the Hamilton of Milwaukee, and may be provisionally 

 placed in the same category. An enlarged view of the surface 

 markings of one of these plates is given in Plate III, Fig. 2. 

 From the last-named locality and horizon are obtained a number 

 of peculiar plates, possibly representing the dorsomedian of un- 

 known forms, two such being illustrated in Plate I, Figs. 2 and 

 6. Aspidichthys, Phyllolepis, and possibly also Holonema are 

 represented in the European Devonian, and one species from the 

 Middle Devonian of Manitoba is doubtfully referred to Aspidich- 

 thys (A. f notabilis) by Whiteaves. 



Order CTENODIPTERINI. 



Body fusiform, without dermal armor. Skeleton and chond- 

 rocranium partially ossified, skull autostylic, premaxillae and 

 maxillae absent. Cranial roof bones small, and, like the squam- 

 ation, with or without ganoine investment. Nostrils inferiorly 

 situated; jugular plates present or not. Tail heterocercal or 

 in some forms apparently diphycercal (gephyrocercal). Anal 

 fin always distinct, the remaining median fins either discon- 

 tinuous or becoming coalesced. Paired fins acutely lobate. 

 Secondary pectoral arch consisting of an ossified supraclavi- 

 cular and clavicle; pelvic arch present. Dentition consisting 

 of large tritoral dental plates supported by the palato-ptery- 

 goid and splenial bones; a marginal series of teeth above and 

 below also sometimes present, but never any vomerine teeth. 



The structure of this singularly interesting order of Palaeozoic 

 fishes has been investigated in minute detail by Hugh Miller, 



* The generic name being preoccupied, Oestophorus has been proposed as a 

 substitute by S. A. Miller. The sense in which these terms are employed is in each 

 case purely provisional. 



