54 PISCES CLASS I 



head, where rudiments occur ; a series of large scutes along the ventral border. 

 L. p'ohlematicus, Traq. Upper Silurian , Lanarkshire. 



Family 2. Euphaneropidae. Smith Woodward. 



As Birkeniidae, hut no series of branchial openings. Upper Devonian. 



Euphanerops, Sm. Woodw. E. longaevus, Woodw., the type species, is the 

 only known Ostracophore which displays traces of the axial skeleton of the 

 trunk. Upper Devonian ; Canada. 



Order 3. ASPIDOCEPHALI. Brandt.^ 



[Osteostraci, Eay Lankester.) 



Exoskeleton consisting of calcifications, partly tuith lone cells ; each plate in the 

 head region comprising three superposed layers — an inner laminated layer with 

 spindle- shaped hone cells; a relatively thick middle layer of polygonal cancellae which 

 may he more or less solid though with a coarse retiadation of large vascular canals ; 

 also an outer hard layer of vaso-dentine. Orhits close together. iYo paired 

 appendages. 



Fainily 1. Ateleaspidae. Traquair. 



Head shield rounded or tapering in front, abruptly truncated behind, consisting 

 of loose plates. Body covered with quadrangular scales which are deepened on the 

 flank. Tail heterocercal. Upper Silurian. 



Ateleaspis, Traq., from the Upper Silurian Passage Beds of Seggieholm, 

 Lanarkshire. The tubercles of the head region are fused together into small 

 plates. The genus may be a link between the Heterostraci and Osteostraci, 

 but the microscopical structiire of the armour agrees with that of the latter. 



Family 2. Oephalaspidae. Agassiz. 



Head shield rounded or tapering in front, abruptly truncated behind ; inter orbital 

 piece firmly fixed ; an ornament of rounded or stellate tubercles. Dermal sense organs 

 leaving no impressions on the exoskeleton. Body covered with quadrangular scales, 

 which are deepened on the flank. One small dorsal fin ; tail heterocercal. Upper 

 Silurian to Upper Devonian. 



Cephalaspis, Ag. (Eurephalaspis, Hemicyclaspis, Zenaspiis, R. Lank.), (Figs. 



Fig. 106. 

 Cephalaspis lyelli, Ag. Lower Old Red Sandstone ; Ai-broath, Scotland. 1/2 nat. size (after Eay Lankester). 



106, 107). Postero-lateral angles of shield more or less produced into pointed 

 cornua, external to a small flexible flap on each side, which is stiffened by 



^ Huxley, T. H., On Cephalaspis and Pteraspis (Quar. Journ. Geol. See. vols. XII., XIT., XVII.), 

 1856, 1858, l^Ql.— Lankester, E. R., The Cephalaspidae (Monogr. Palaeont. See), 1867-69.— 

 Schmidt, Friedr., Verhandl. k. riiss. mineral. Ges. 1873 and 1886 ; and Bnll. Acad. Imp. Sci. St. 

 Petersb. 1894. — Rohon, J. V., Die obersilurischen Fische von Oesel I. (Mem. Acad. Imp. Sci. St. 

 Peters!)., ser. 7, vol. XXXVIII.), 1.892 : also Bull. Acad. Imp. 1893. 



