Sub-Grade 2. OSTRACODERMI. 



This is a merely provisional group, in which are gathered some 

 of the oldest and least satisfactorily known Palaeozoic fish. So 

 aberrant are they in structure that their very right to be placed 

 among the Pisces at all has been disputed (Cope, A. S. Woodward 

 [503, 505]). Cope, indeed, suggested that they are allied to the 

 Cyclostomes. While the relationship to each other of the various 

 orders included in this Division remains almost as obscure as ever, 

 the affinities of the Pteraspidomorpbi to the Elasmobranchii, through 

 the Coelolepidae, seem to be now well established by the researches 

 of Traquair. On the other hand, the presence in the Cephalaspido- 

 morphi and Pterychthiomorphi of true bone in the exoskeleton, and 

 of fin-rays and fulcra having considerable resemblance to the lepi- 

 dotrichia and fulcra of the Osteichthyes, points doubtfully to some 

 connection with the latter. The fact that true jaws have not yet 

 been detected has little weight, since no remains of any part of the 

 endoskeleton have been preserved in these very ancient fossils 

 (except, perhaps, in the tail of some Anaspida, p. 206). The 

 Ostracodermi have very few characters in common beyond the 

 possession of a single dorsal fin, a heterocercal tail, and a 

 tendency, so to speak, for the head and trunk to become enclosed 

 in dermal shields. In all, except the Anaspidn, the lateral-line 

 system has been described. The mouth is ventral, and some sort 

 of branchial opening can generally be made out. No very definite 

 traces of nasal or of auditory organs can be seen, though there are 

 occasional vague indications of their presence. Paired orbits and 

 a median pore or depression, probably due to a pineal organ, are 

 generally, if not universally, present. It is to be noted that normal 

 paired limbs have never been found in any member of the group ; 

 but in some the body expands at the sides into a longitudinal fold 

 (Coelolepidae), or distinct flap (Cephalaspidae), or jointed appendage 

 (Pterychthyidae), which may represent a pectoral limb. Of a 

 separate pelvic limb there is not the slightest trace. 1 



1 The very interesting fragments from the Lower Old Red Sandstone described by 



Traquair under the name Far ml I in I nU, rculata possibly belong here. This fossil has 

 polygonal scutes and ring- like centra. Similar scutes have been found in the 

 Silurian Ludlow bed (A. S. Woodward). 



194 



