538 PBO]?. T. W. BRIDGE ON THE MESIAL 



resembles the second *. In all the remaining trisegmental 

 elements (PL XXI. fig. 2) the proximal segment (^.s.)is a some- 

 what dagger-shaped bone, slightly broader at its distal extremity 

 where it is tipped with cartilage, but pointed and completely bony 

 at its inner end ; and, moreover, presents no trace of the cha- 

 racteristic lateral longitudinal ridges which in most Teleosts 

 separate the elevator and depressor muscles of the fin-rays. 

 The mesial segments (m.s.), on the other hand, are short, some- 

 what hour-glass-shaped bones with cartilaginous extremities, 

 while the distal segments (d.s.) are invariably small cartilaginous 

 nodules. The three segments of each complete radial element 

 are in ligamentous connexion with one another, and also with 

 the corresponding segments of contiguous elements. 



In one important feature the radial elements of A7nia differ 

 greatly from those oiPolyodon, Acipenser, and the Elasmobranchs, 

 and resemble the corresponding structures in Lepidosteus, and 

 in those Teleosts in which the trisegmental type of radial 

 element exists. The proximal segments are widely separated 

 from one another, and the only connexion between them is the 

 median vertical sheet of fibrous tissue in which they are 

 imbedded ; but the mutual relations of the mesial and distal seg- 

 ments are nevertheless such that the various radial elements 

 afford one another mutual support, and two of them contribute 

 to the support of each dermal fin-ray. Thus, each mesial seg- 

 ment is inclined backwards at an angle with the proximal 

 segment and its distal or hinder extremity articulates with, or at 

 all events rests upon, the anterior margin of the distal extremity 

 of the proximal segment of the next succeeding radial element, 

 while each distal segment is in part supported by its own mesial 

 segment and in part by the anterior or upper margin of the mesial 

 segment of the next radial element. Hence, as each distal 

 segment carries a soft fin-ray, it follows that the latter is sup- 

 ported partly by the distal segment of the radial element to 

 which it normally belongs, and partly also, but indirectly, by the 

 mesial segment of the next succeeding element. All the fin-rays 

 are of the soft multiarticulate kind, and each is cleft basally for 

 the reception of the distal segment of a radial element. 



The numerical disproportion between the radial elements and 



* Immediately behind the forty-ninth, and in close relation witli it, there is 

 a vestigial element, consisting of a proximal segment only and without a dermal 

 ray. 



