PTERICHTHYOMORPHI 207 



(Patten [328]). In Asterolepis and Pterichthys the tail is covered 

 with imbricated scales, smaller scales extend in rows on to the fins, 

 and powerful fulcral scales lie along the mid-dorsal line and the 

 anterior edge of the fins. Bothriohpis has an almost naked caudal 

 region. The lateral-line system is represented by grooves running 

 round the cephalic shield, passing ventrally in front of the mouth, 

 and down the sides of the trunk- cuirass on to the tail. The course 

 of these grooves on the head is very different from that pursued 

 in typical Pisces. 



In front of the mouth below are placed two dermal ' maxillary ' 

 plates, with toothed edges, which seem to represent jaws. On the 

 outer edge of each is a notch, perhaps indicating a nostril (A. S. 

 Woodward [503]). The mouth is limited behind by two small 

 semilunar plates at the edge of the ventral trunk-shield. The two 

 orbits are situated close to each other near the middle of the 

 cephalic shield. Each is protected by a thin, convex, bony plate ; 

 and between them is a free quadrangular plate, with a deep inner 

 excavation, possibly for a pineal organ. 



The plates composing the cephalic shield do not agree in 

 number or disposition with the normal structure of the skull of 

 a Teleostome. Their arrangement, and also that of the plates 

 forming the trunk-cuirass, will best be understood by consulting 

 Fig. 181. A large plate on each side of the head appears to have 

 been movable and with a free edge it may represent an operculum. 

 The one median and four paired plates composing the ventral trunk- 

 shield bear a remarkable resemblance to the plastron of the 

 Coccosteids (p. 261). According to Patten [328] the cuirass had 

 a free membranous edge behind, leaving a branchial opening 

 between it and the trunk. But the position of the branchiae, and 

 indeed their very presence, has not yet been definitely determined. 

 Probably they were within the cephalic shield. 



Articulating near the front edge of the cuirass are two most 

 remarkable two-jointed, freely movable appendages, with serrated 

 anterior edge. They are completely encased in long plates. The 

 cavity inside the appendage communicates with that of the body 

 by an aperture in the cuirass. Most authors compare these 

 appendages to pectoral fins (Fig. 181). 



So striking is the resemblance between the Pterichthyomorphi 

 and the Coccosteomorphi, not only in the general shape of the body 

 and the development of the cephalic and trunk shields, but also 

 in the minute structure of the dermal skeleton, that they were for 

 a long time associated by the older authors. Yet the two groups 

 differ in several fundamental characters. The presence of pectoral 

 appendages in the former, their absence in the latter ; the presence 

 of pelvic fins in the latter, their absence in the former ; and above 

 all, the possession of recognisable upper and lower toothed jaw-bones 



