1921.] ■ S. L. HoRA : Absence of fin^ in fishes. 29 



specimen was collected by Mr. Hamid Khan in the Ravi River 

 near Lahore. He noticed this abnormality and presented the 

 specimen to the Government College Museum, Lahore. I am in- 

 debted to Prof. George Matthai for lending me the specimen from 

 the above Museum and for allowing me to dissect it for study 

 of the skeleton and the musculature of the abnormal region. 



The pectoral fin in the genus Rita is a characteristic structure, 

 as it is provided with a very strong spine. The spine is almost as 

 long as the length of the pectoral girdle. The fish does not seem 

 to have suffered as regards its size ; its length including the length 

 of the caudal iin is 326 cm., which is the average length of the 

 species at Lahore. 



Correlated with the absence of the fin, the various muscles, 

 associated with it are either absent or have undergone considerable 

 degeneration. The abductor and adductor muscles {e, /) are total- 

 ly absent. Of the muscles (a, b, c, d), which control the move- 

 ments of the spine, a and d are not represented, while the other 

 two, b and c, are greatly reduced. An accessory nodule of bone 

 was found in the course of the greatly reduced muscle of the ab- 

 normal side. I am unable to understand its significance as the 

 structure is not represented in the muscle of the opposite side or 

 in other normal specimens. 



The shoulder girdle of the abnormal side is considerably short- 

 er in width than that of the opposite side. The primary girdle 

 (pg.), consisting mainly of the scapular and coracoid bones, is 

 either altogether absent on the abnormial side or has so fused with 

 the secondar}^ investing bones of the girdle as to be quite indistin- 

 guishable from them. Consequently the canals (t', c") for the pas- 

 sage of the muscles of the spine are wanting, and this to a certain 

 extent may account for the absence or degeneration of the muscles. 

 The deep groove (a.g.), for the articulation of the condyle at the 

 base of the spine, formed by the cubito-humeral process (h) and 

 the clavicular element (s.g.) is represented on the abnormal side 

 onl}^ by a notch (n) in the cubito-humeral process. 



The degeneration of the muscles of the right side and the 

 abnormalities in the skeleton of this region indicate that the ab- 

 sence of the fin is not due to any recent accident or injury. It 

 seems quite probable that all the abnormalities noticed above are 

 the direct or indirect result of the absence of the primary shoulder- 

 girdle of the abnormal side ; this will also account for the absence 

 of the right fin. The musculature was probably affected second- 

 arily, while the reduction in the secondary girdle is chiefly due to 

 the disuse of the associated structures. 



In almost all such abnormalities, it has been pointed out that 

 the growth of the fish is not effected by them. It is only after 

 careful examination that these abnomal forms can be separated 

 from normal specimens netted with them. 



Abnormalities of this nature have been regarded as con- 

 genital variations. Willey {op. cit.) regards thern as mutations and 

 comes to the conclusion " that the presence or absence of such deep- 



