VERTEBRATES. 229 



obtains in forms of teeth of opposite jaws of Tertiary and existing 

 Myliodonts. 



The affinities of the Copodi are certainly with the Myliodonts, the 

 single row of teeth, spanning the jaws like paving flags, finding a 

 somewhat intimate repetition in the still existing genus Aetobatis. 

 There are, however, other characters that offer marked contrasts in 

 contradistinction of the ancieijt and the latter modern representa- 

 tive, and which probably should be interpreted as possessing an 

 importance even of greater consequence than generic. The charac- 

 teristics possessed in common with the genus Psammodus, Agass., 

 show unmistakable family relationship with the latter, while structur- 

 ally and in the character of the basal portion of the teeth there is 

 even striking contrast to the features noted in typical Myliodonts. 

 We are, therefore, of the opinion that the family distinctions of the 

 Pala-ozoic teeth here alluded to are paramount and should be so 

 recognized ; hence the family designation Psammodontidfe, which was 

 bestowed by Dr. de Koninck on this group of ancient selachian re- 

 mains, is here adopted. 



The genus appears to be restricted to the earlier groups of Car- 

 boniferous formations both in Europe and America. 



COPODUS VAN HORNII, St. J. and W. 



PI. XX, Fig. 2, 3. 



Teeth representing the forms pertaining respectively to the upper 

 and the lower jaws, and which attain medium size. 



Maxillary teeth liugulate in outline, very gently arched from 

 within outward. Lateral borders nearly straight, gradually con- 

 verging and angularly rounded into the anterior margin, posteriorly 

 terminating in a produced spur, the basal portion projecting in a 

 widish rim beyond the coronal limits, inferiorly inbeveled and 

 rounded to the inferior surface ; anterior margin regularly and mod- 

 erately arched, the coronal band forming a coping constituting one- 

 fourth or one-third of the depth, and well defined from the exca- 

 vated basal portion ; the posterior margin approximately conforms 

 to the curvature of the opposite margin, and is similarly marked, 

 the channeled basal portion in both instances irregularly vertically 

 pitted. The coronal region is moderately arched transversely, flat- 

 tened along the median line, with a lunate depressed triturating 

 area reaching back from the anterior margin a greater or less dis- 



