1897.] 



iPKBSBKOE or RIBS IN POLYODON. 



n'^ 



In one or two works (Huxley's 'Anatomy of Vertebrated Animals,' 

 p. 139, and the ' Text-book of Zoology' by Boas [English transl. by 

 Kirkaldy and Pollard, p. 361]) statements occur which imply the 

 absence of ribs in some existing Glanoids, and, as there is no 

 question as to their presence in Acijjenser, Pohjpterus, Amia, and 

 Lepidosteus, the statements in question presumably refer to Poly- 

 oclon. In others, again, no mention is made of the presence or 

 absence of ribs in Polyodon, altliough references to the corre- 

 sponding structures in Acipmiser are sufficiently frequent. 



These quotations are sufficient to prove that the question of the 

 presence or absence of ribs in Polijodon has been the subject of 

 several contradictory statements, and is still involved in no little 

 obscurity. 



'hv>- 



^^2^Z^I:^ZtCi . 



•7uo. 



Lateral view of a portion of the anterior section of the vertebral 

 column of Polyodon folium. 



bv., inferior vertebral arch or " basi- ventral " cartilage ; bd., superior vertebral 

 arch or " basi-dorsal " element; he, lii.T:;nal canal; id., neural intercalary 

 or interdorsal element ; iv., inter-ventral or htemal intercalary cartilage ; 

 Iff., ligament; nc, notochord ; n.s., neural spine ; ;■., rib. 



"With regard to the actual facts of the case there can be no doubt 

 as to the presence of ribs in Polyodon. In a specimen about 

 40 inches in length I found a series of fifteen simple, filament-like, 

 cartilaginous ribs, commencing anteriorly near the point where the 

 superior vertebral arches first commence distinctly to differentiate 

 themselves from the continuous cartilaginous mass which is formed 

 by the fusion of certain of the anterior arches with the chondro- 

 cranium, and terminating a little posterior to the anterior half of the 

 pre-cloacal section of the vertebral column. The third to the fifth 

 ribs, inclusive, are perhaps the longest, being about 7 mm. in length 

 and about 1 mm. in thickness. The first and second are a trifle 

 shorter, but behind the fifth the ribs rapidly decrease in size until 

 the hindermost of the series are less than 1 mm. in length, being, in 

 fact, simple nodules of cartilage. Each rib (fig., r) is rather loosely 

 connected by ligament (Ig.) with the hinder extremity of a longi- 



