46 ' RAYS. 



interspaces of the main tubes. These tubes, at their origin, present 

 a diameter of 3^ of an inch, with interspaces equal to those of their 

 diameters. Tlie smallest branches into which they are resolved, and 

 which are lost in the clear enamel-like external stratum of the 

 tooth, have a definable diameter of 37^ of an inch. This external 

 layer is thickest at the upper surface of the tooth, and the boundary 

 between it and the tubular substance of the tooth, is here defined 

 by a stratum of calcigerous cells. Some of the fine peripheral 

 branches of the calcigerous tubes unite and form loops, the con- 

 vexity of which is turned towards the cells, and from which the finer 

 branches proceed to communicate with the superficial calcigerous 

 cells. 



17. Myliobafes. — The modification of the plagiostomous type of 

 teeth, for the purpose of crushing the alimentary substances, is most 

 complete in this genus. A view of this armature of the mouth, as 

 seen from behind in the Myliobates Aquila is given PI. 25, fig. 1. Both 

 jaws are covered with a pavement of broad teeth, with a flat grinding 

 surface, vertical and finely undulated sides, by which contiguous 

 teeth are joined together as by a suture, (PI. 27, c) and a base 

 divided into a number of small parallel longitudinal ridges. 



The entire phalanx of dental plates of the upper jaw describes 

 the segment of a circle. In the Myliobates marginata (subg. Rhi- 

 noptera of Kuhl), the teeth are hexagonal, the middle row being the 

 broadest, and the lateral ones diminishing as they recede ; the 

 outermost are mostly pentagonal. I have seen an example of the 

 jaws of this subgenus of Myliobates, in which one of the rows next 

 the median was subdivided into two unequal series, as represented in 

 fig. 2, pi. 25. In Myl. Jussieui fZygobates, Agass.), the three middle 

 rows of teeth are much broader than the two marginal rows. In My- 

 liobates proper, the median row itself forms the principal part of the 

 dental covering of the jaws, which is bordered by three narrow rows of 

 teeth, having the same antero-posterior extent as the median plates. 

 Finally, in Myliobates Narinari (^Aetobatis, Miiller) , the small marginal 

 teeth have entirely disappeared, and the jaws support a single row of 

 broad dental plates. In the upper jaw, these plates are arched, with the 

 convexity turned forwards ; in the lower jaw, they pass straight 

 across, with the extremities only a httle bent backwards. The jaws 



