COCCOSTEOMORPHI 259 



tiated ginglymoid joints. The trunk tapers into a tail, quite or 

 very nearly diphycercal. There is a single dorsal fin. Of pectoral 

 tins there is no definite trace, and the pelvic fins are only known 

 from rare remains of the endoskeleton. The body was apparently 

 naked in most cases, though scattered tubercles have been found in 

 Selenosteus (Dean [109]), and thin cycloid scales are stated to 

 occur in Coccosteids (Jaekel [244]). The structure of the dermal 

 bones is very like that of Asterolepis. They have, in the more 

 primitive genera, a tuberculated surface ; in others, such as 

 Titanichthys, the ornamentation has gone, the bones having 

 probably sunk below the skin. The dermal plates are formed of 

 true bone with bone-cells, an internal lamellated layer, a middle 

 vascular trabecular layer, and an external layer of denser substance. 

 Neither typical ganoine nor cosmine, neither denticles nor even 

 dermal fin -rays have been described. An elaborate and fairly 

 normal system of lateral-line canals is indicated on the cranial 

 shield by grooves ; they extend on to the lateral, and even on to 

 the ventral, regions of the body -cuirass. In the Anarthrodira 

 (Dean [109]) the canals are sunk below the surface of the bones. 



The endoskeleton is scarcely known except in Coccosteus (Fig. 

 232). The notochord appears to have been persistent and uncon- 

 stricted ; no remains of centra are found. There are no ribs, but 

 well-developed haemal and neural arches are present. The dorsal 

 fin is supported by a series of two -jointed radials attached to and 

 corresponding with neural arches. A skeletal plate possibly belongs 

 to an anal fin (Fig. 232). Two basal pieces, with traces of radials, 

 seem to represent a pelvic girdle and pelvic fins. These endoskeletal 

 structures are of calcified cartilage. 



The dermal plates are best known in Coccosteus (Pander [312], 

 Traquair [454, 457], A. S. Woodward [505]). An anterior ethmoid 

 (rostral) between the nares is followed by a ' posterior ethmoid ' 

 or pineal plate. There is a large median occipital behind ; paired 

 ' preorbitals ' ( = prefrontals + frontals ?) and ' centrals ' ( = parietals) 

 cover the skull above ; circumorbital plates surround the eye ; 

 ' postorbitals ' ( = postfrontals) and ' marginals ' ( = pterotics) cover 

 the sides ; ' external occipitals ' (supratemporals) complete the shield 

 behind (Fig. 229). Toothless premaxillae and maxillae (suborbitals) 

 are found on the upper jaw ; the latter expands behind to cover 

 the cheek, and bears on its posterior edge a crescentic plate, which 

 is supposed to have supported a movable operculum. As indicated 

 above, these plates can, with more or less certainty, be homologised 

 with the bones of more normal Osteichthyes. 



The solid trunk-cuirass may represent a much-modified dermal 

 pectoral girdle (Figs. 229, 229A). The ventral shield consists of 

 four paired and two median plates, of which the anterior is compared 

 to an interclavicle. In front are two elongated inter-lateral bones 



