458 PROF. ST. GEORGE MIVART ON THE 



which do not coalesce together at all, are not attached to any basal cartilage, and have 

 no pelvic cartilage. There are eighteen such radials, each once segmented; and the 

 distal segment of the most preaxial radial is longitudinally divided into two closely 

 applied rods. I suspect, from the form of the distal ends of the distal segments, that 

 other small cartilages were appended to their apices. There also appears to be some- 

 thing wanting between the two segments of each radial '. 



POLYPTERUS BICHIR. 

 Dorsal Fin (Plate LXXIX. fig. 6). . 



This fin is supported by radials which give off on one side small secondary rays, 

 proceeding dorsad and postaxiad. 



Venteal Fin and Anal Fin (Plate LXXIX. figs. 7 & 8). 

 These two fins, when compared together, exhibit much similarity. In the ventral 

 fin the fin-rays are attaclied to the apices of four cartilaginous radials, while those of 

 the anal fin are borne by six essentially similar radials. Let but the two postaxial 

 radials of the anal abort, and let the apices of its two or three preaxial radials elongate, 

 coalesce, and become segmented off", and the form of the anal fin would be changed into 

 that of the ventral one. 



ON THE NATURE AND HOMOLOGIES OF VERTEBRATE LIMBS. 



The examinations I have been so kindly permitted to make have partly been under- 

 taken with a view to two special questions, which are : — 

 (1.) What is the nature of vertebrate limbs generally? 

 (2.). What is the relation of piscine to other limbs'? 

 To answer these questions satisfactorily, replies must be found to four questions of a 

 subordinate nature. These are : — 



A. Are the paired limbs structures of a nature distinct from that of azygos fins ? 



B. Are paired limbs essentially axial structures, which have become more or less 



detached from the skeletal axis, or peripheral structures which have become 

 secondarily more or less connected with it'? 



C. What is the probable nature of the parts to which they are attached, i. e. of 



the limb-girdles 1 



D. Can we yet accurately determine the line of genesis of the cheiropterygium 1 



' Since this paper was delivered, I have received from Mr. James K. Thacher a paper on the ventral fins of 

 Ganoids (from the Transactions of the Connecticut Academy, vol. iv.), in which he describes and figures the 

 ventral fin of Polyodon. I find it has small apical cartilages, while at the point of segmentation a process is 

 given off dorsad from a greater or less number of the radials — a process which may be called a sort of separate 

 iliac process for each such radial. In ilr. Thacher's specimen also a few of the radials coalesce proximally, 

 but there is stiU no continuous basal cartilage as in the ventral-fin skeleton of Sharks. 



