RELATIONSHIP OF AMMOCCETES TO OSTRACODERMS 353 



frontal nose-organ; therefore, Patten looks upon the nose and the 

 two lateral eyes of the Osteostraci as a complex median eye, regard- 

 less of the fact that the median eyes already existed. 



Every atom of evidence Patten has brought forward, every new 

 fact he has discovered, confirms my position and makes his still more 

 hopelessly confused. Keep the animal the right side uppermost, and 

 the evidence of the rocks confirms the transition from the Pakeo- 

 stracan to the Cyclostome ; reverse the surfaces, and the attempt to 

 derive the vertebrate from the palaaostracan becomes so confused and 

 hopelessly muddled as to throw discredit on any theory of the origin 

 of vertebrates from arthropods. For my own part, I fully expect 

 that appendages will be found not only in the Cephalaspidse but also 

 in the Pteraspidse, and I hope Patten will continue his researches 

 with increasing success. I feel sure, however, his task will be much 

 simplified if he abandons his present position and views the question 

 from my standpoint. - 



Summary. 



The shifting 1 of the nasal tube from a ventral to a dorsal position, as seen 

 in Ammoccetes, is, perhaps, the most important of all clues in connection with 

 the comparison of Ammocoetes to the Palfeostracan on the one hand, and to the 

 Cephalaspid on the other ; for, whereas the exact counterpart of the opening- 

 of such a tube is always found on the dorsal head-shield in all members of the 

 latter group, nothing of the kind is ever found on the dorsal carapace of the 

 former group. 



The reason for this difference is made immediately evident in the develop- 

 ment of Ammocoetes itself, for the olfactory tube originates as a ventral tube — 

 the tube of the hypophysis — in exactly the same position as the olfactory tube of 

 the Pala^ostracan. and later on .in its development takes up a dorsal position. 



In fact, Ammocoetes in its development indicates how the Palwostracan 

 head-shield became transformed into that of the Cephalaspid. 



In another most important character Ammoccetes indicates its relationship 

 to the Cephalaspid A3, for it possesses an external skeleton or head-shield composed 

 of muco-cartilage, which is the exact counterpart of the so-called bony head- 

 shield of the latter g-roup ; and still more strikingly the structure of the 

 cephalaspidian head-shield is remarkably like that of muco-cartilage. In the one 

 case, by the deposition of calcium salts, a hard external skeleton, capable of 

 being preserved as a fossil, has been formed ; in the other, by the absence of 

 the calcium salts, a soft chondro-mucoid matrix, in which the characteristic 

 cells and fibrils are embedded, distinguishes the tissue. 



The recognition that the head-shields of these most primitive fishes were 

 not composed of bone, but of muco-cartilage, the precursor of both cartilage 

 and bone, immediately clears up in the most satisfactory manner the whole 



2 A 



