192 DR. "VV. a. RIDEWOOD ON THE CEAJSriAL OSTEOLOGY OP 



cartilage-bone of considerable size which may be taken to 

 include the glossohyal (endosteal part) and the first basi- 

 branchial. The ceratohyal fits on to the side of this compound 

 bone. The urohyal is a large vertically disposed plate of bone, 

 having a thin posterior edge. It is immovably fixed, and is 

 united not only with the bone just mentioned, but also with the 

 under surface of the anterior two-thirds of the second basi- 

 branchial. The usual paired ligament so characteristic of the 

 urohyal is not recognizable. The dentigerous membrane-bone is 

 narrow, bears some forty, more or less vestigial teeth, and 

 extends back over nearly the whole of the second basibranchial. 

 The third basibranchial is a small flat plate of bone. 



The first hypobranchials are not rods of bone projecting 

 freely backward and outward from the junction of the first and 

 second basibranchials, and setting the anterior ends of the first 

 ceratobranchials at some distance from the mesial structures, but 

 they are quite short bones, closely applied to the sides of the 

 second basibranchial in such a way that the antero-internal point 

 of the first ceratobranchial nearly touches the side of the second 

 basibranchial. In a somewhat similar way the anterior ends of 

 the second ceratobranchials come close to the mesial series of 

 bones; but in this case the hypobranchials project downward from 

 the sides of the posterior end of the second basibranchial, and 

 are fused witli a pair of tendon-bones similar to those of 

 Notopterus (see page 206). (It must be borne in mind, however, 

 that the first, second, and third hypobranchials of Notopterus are 

 quite normal, and that the paired tendon-bones are readQy 

 separable from the antero-inferior ends of the second hypo- 

 branchials and the postero-inferior surface of the second basi- 

 branchial.) The reasons for not considering the whole bone as 

 the equivalent of the tendon-bone of Notopterus, and regarding 

 the second hypobranchial as wanting, is that the upper part of 

 the bone has the appearance of a cartilage-bone, which is not 

 the case in Notopterus ; that the third hypobranchials are, 

 though small, downwardly directed ; and that it strikes one as 

 improbable that the second hypobranchials should disappear 

 absolutely while the first and third remain. 



The first pharyngobranchial is present, and has the form of a 

 small triangular plate of bone, horizontally placed, and in 

 contact with the antero-internal edge of the first epibranchial 

 and the anterior edge of the second pharyngobranchial. There 



