158 JDr. R. H. Traquair — On Brepanaspis, 



which pass into the median fulcra of the ventral margin of the tail 

 and caudal fin ; these fulcra being at first very stout and prominent, 

 but becoming further back proportionally more slender and oblique. 



The above is a sketch of the configuration of Drepanaspis and of 

 the parts forming its exoskeleton, so far as I have been able to 

 elucidate the subject after ten years' careful hoarding up of material. 



Observations. — It will, I think, be acknowledged -that Drepanaspis 

 is a creature, the details of whose structure as described above form 

 an important addition to our knowledge of the extinct Ostracodermi 

 of the Silurian and Devonian periods. Its resemblance to the still 

 imperfectly known Psammosteidse cannot be gainsaid, nor can its 

 affinity to the Coelolepidse on the one hand and to the Pteraspidae on 

 the other be doubted, and consequently I have in my memoir on 

 the Scotch Silurian fishes included all four families in a new 

 and enlarged conception of the Heterostracous division of the 

 Ostracodermi. 



Looking at Drepanaspis by itself we have a creature whose hard 

 parts are entirely dermal, and in which no traces whatever of any 

 endoskeleton can be found. 



The mouth is a ti'ansverse slit, which shows no teeth, nor any 

 jaws properly so called, and therefore affords an apparent support to 

 the agnathous theory of the Ostracodermi. But, I repeat, we know 

 nothing of the original cartilaginous endoskeleton, and consequently 

 to believe that the fish while living had no cartilaginous repre- 

 sentative of the Meckelian element is surely an assumption, while 

 to argue therefrom any special affinity to the Lampreys, so utterly 

 different in form, seems to me to be a non sequitiir. Here it may be 

 observed that the condition of the nasal organ does not afford us any 

 help in this question, as no external nostril, either single or double, 

 has been observed in Drepanaspis or in any other Ostracoderm. 

 The only Palfeozoic fish which is apparently monorhinal like the 

 Lamprej'S is Palcsospondylus. 



There is certainly no shoulder girdle in Drepanaspis, nor are 

 there paired fins, at least from the functional standpoint. But the 

 prominent postero - lateral angles of the carapace are obviously 

 equivalent to the lateral expansions in the Coelolepidee, which I have 

 interpreted as pectoral fin-folds. If this idea is right, we have the 

 parts representing the pectoral fins in Drepanaspis replaced by 

 lunyielding plates. 



Dr. Smith Woodward, in his review of my Silurian memoir, dis- 

 ao-rees with this view on two grounds. 



First, because thei'e is no line of demarcation between the fin-like 

 expansions of Thelodus and the rest of its body, no change in the 

 character of the dermal armature at that part, and no signs of 

 flexibility. 



But the very same things may be said of the caudal fin of Thelodus 

 and of Drepanaspis, and surely no one would deny that it, at least, 

 is a fin. In Drepanaspis, as we have seen, the expanse of the caudal 

 fin is covered with scales preciselj^ similar in shape and in sculpture 

 to those of the body prolongation, between which and the fin 



