1896. SHARKS AS ANCESTRAL FISHES. 247 



features which could be attributed to Plenvacanthus are certainly- 

 present among recent sharks — e.g., the possession of a spiracle, and of 

 a notochordal skeleton. 



A study of the second Palaeozoic shark, Chondrenchelys, 

 Traquair, so far as our present knowledge goes, affords just as little 

 satisfactory evidence as to the primitive characters of primaeval 

 sharks. This form may indeed have been rather of a pleuracanth 

 type, with its elongate body and tapering tail ; nevertheless, it was 

 certainly well differentiated in its skeletal characters, possessing well- 

 marked vertebral centra and arches. 



It is accordingly to the third and oldest of these ancient sharks 

 that we have finally to turn for more definite suggestions as to the 

 primitive characters of Elasmobranchii. Cladoselache has already 

 been mentioned several times in the pages of Natural Science in 

 connection especially with the primitive characters of its fins. Its- 

 remaining structures, however, are found to prove equally important 

 from the morphological standpoint ; and in the present connection 

 a summary of its general characters may be given. 



Cladoselache is at the present time to be looked upon as representing 

 a group of Palaeozoic sharks included in a single genus (perhaps twa 

 genera), and so far as the present writer is aware, in about a dozen 

 species. These are relatively small in size, varying from two to six 

 feet in length. They were first brought into notice about i888 by 

 the late Professor J. S. Newberry, of Columbia College, New York,, 

 and they have subsequently been described by Professor E. W. 

 Claypole, Mr. Arthur Smith Woodward, Dr. Otto Jaekel, and the 

 present writer. The material from which these fossils were originally 

 known has now been added to the collection of Columbia College ;. 

 but an abundant material, including several dozen complete specimens,, 

 some of which are admirably preserved, is now in the private 

 collections of Dr. Wm. Clark, of Berea, Ohio, and Rev. Wm. Kepler, 

 of New London, to both of whom belongs the credit of its discovery .- 



The Cladoselachidae have .been found only in the Waverly 

 Sandstone (at the base of the Lower Carboniferous) of Ohio ; they here 

 occur in heavy, oblong concretions, which are usually obtained when^ 

 weathered out of the precipitous stream-margins. A concretion con- 

 tains usually the entire fish, shown in nearly every case in ventral. 

 or dorsal aspect. The only part of the animal which was subject 

 to widely varying conditions during the period of fossilisation was 

 the body-width ; the digestive tract and the visceral cavity seem here 

 to have been considerably inflated with gases of decomposition, and 

 justly account for the present position of the fossil. The antero- 

 posterior dimensions of the fish are found to be constant in the same 

 species. 



A careful series of measurements of well-preserved fossils- 

 has enabled the present writer to complete a restoration which he 

 believes to represent Cladoselache in three aspects with fair accuracy,. 



