III. FOSSIL CH I M/ERO IDS: 

 THEIR SIGNIFICANCE IN THE STUDY OF RECENT FORMS. 



The evidence of paleontology in the problem of Chimferoid descent is import- 

 ant, although one must frankly admit that it is still lacking in essential details, for 

 not only are fossil Chimsroids rare, but they occur with but few exceptions m 

 fragmentary^ form. 



An outline of the distribution in time of the genera of Chim^eroids is shown 

 at the bottom of page 134. In this has been omitted reference to the supposed 

 Silurian Chima^roid Didyorhabdus priscus Walcott, for reasons which are stated 

 below. Among the genera given, it will be seen that three, doubtfully Chimaeroid, 

 are Devonian, representing together about 16 species; one, probably a Chimseroid, 

 is Permian, and four are exclusively Jurassic. P^rom this time onward the greatest 

 number of genera flourished in the Cretaceous, representing at least 50 species, and 

 one of these genera, Ischyodus, extends from the Jurassic into the early Miocene. 

 Another, a Cretaceous genus, Callorhynchus, is, as we have seen, represented by 

 half a dozen species at the present time. 



With this plan of distribution in mind, we may summarize our knowledge of 

 fossil Chimseroids with reference especially to their advancing characters. 



THE QUESTION OF A SILURIAN CHIM/EROID. 



Paljeozoic Chima;roids claim evidently our closest attention, and we should 

 consider first of all the question of the "fossil Chimseroids" described by Walcott in 

 1885. At Canon City, Colorado, in the Ordovician (Upper Silurian), the United 

 States Geological Survey obtained a number of narrow, ribbon-shaped fossils which 

 were described by Walcott as Didyorhabdus priscus, and were regarded provision- 

 ally, on account of their general shape and transverse striation, as vertebral 

 columns of a chimaeroid fish.* In spite of the relative abundance of these fossils, 

 however, no Chimaera-like dental plates, spines, or kindred structures were found, 

 a condition the more remarkable since in the matrix there occur innumerable frag- 

 mentary "fish" remains. It is therefore doubtful whether so delicate a structure as 

 the vertebral column of a Chimaeroid would be preserved if no traces were present 

 of associated spines, heavier cartilages, and dental plates. The chimaeroid nature 

 of the fossils, moreover, becomes more doubtful still if they are closely scrutinized. 



*Walcott, it should be stated, refers doubtfully to their cfiimsroid nature. The "correlation is based entirely 



upon the resemblance between the fossil form and the calcified sheath of Ckimrcra moiis/rosn. This resemblance is 



too striking to be passed over, although there are certain differences that render it of less value in classification than 



at first." 



133 



