98 SYSTEMATIC PALEONTOLOGY 



TJiis fragment is evidently one of the terminal costals Avith no jDro- 

 jecting portion of the rib beyond the plate. 



Occurrence. — Aqijia FoKMATIO^T. Aqiiia Creek. 



Collections. — Wagner Free Institute of Science, Johns Hopkins Uni- 

 versity, Maryland Geological Survey. 



Class PISCES. 



Order ELASMOBRANCHII. 



Suborder TECTOSPONDYLI. 

 Family MYLIOBATIDAE. 



Genus MYLIOBATIS Cuvier. 



The following remarks on the crushing pavement-teeth of the great 

 "Eagle-rays," so abundant in the American and European Eocene, are 

 extracted from an important paper on sharks' teeth from the English 

 Eocene by A. S. Woodward.' 



" The dentition of each jaw in this genus [Myliohatis'] comprises 

 large, flattened, hexagonal teeth, arranged in seven antero-posterior 

 series. In very young individuals, the teeth are all approximately of 

 equal size, but quite early in life the median teeth begin to become rela- 

 tively very broad, and as the animal grows, this disproportion of the 

 median teeth gradually becomes greater and greater. When unworn 

 or unabraded, the grinding surface of the teeth is covered with a thin 

 enamel-like layer of gano-dentine, usually marked with antero-pos- 

 teriorly directed striations; but when this layer is removed, the tooth 

 has a punctate appearance, owing to the exposure of the vertical nutri- 

 tive canals traversing the underlying vascular dentine. In naming the 

 fossils, it is thus necessary to take into account the size of the specimen, 

 and remember that the surface markings depend on the state of preser- 

 vation. It is also necessary to note that the dental plate of the lower 

 jaw is flat, while that of the upper jaw curves round the supporting 

 cartilage." 



1 Proc. Geol. Assoc, vol. xvi, ISCO, p. 3. 



P 



