1910.] CAUDAL FIX OF T.'IE TELEOSTOMI. G03 



MuR.y.xA HELEXA. (MuiseuicUo.) 



The caudal fin of the llonian Eel is essentially similar to tliat of 

 the Congei-, and attention need only be drawn to the peculiarities 

 presented. The hypural bones are attached to the last centrum. 

 The spur-like process of the last neural arch, mentioned in Com/er, 

 is more pronoiniced and fuses with a posterioi'ly dii'ected process 

 of the first hypural bone. The posterior neural ai-ches are very 

 simple, having no spine, and the closely aggregated ladials of the 

 dorsal fin are in close contact with them. The liajmal arches very 

 closely resemble the neural arches : the penultimate vertebra does 

 not bear a hypural bone. 



Myrus vulgaris. (Mura?nida\) (Plate XLVIII. fig. 13.) 



The caudal of this fish is again similar to the preceding types. 

 Only one hypural is attached to the last centi'um, which is quite 

 indistinguishable, appearing oidy as the proximal end of the 

 hypural bone : only one hypural may be said to be attached to 

 the penultimate vertebra, though it is clear that this has been 

 formed by the coalescence of the distal ends of the arms of the 

 U-shaped structure in other geneia. The haimal arches of the 

 posterior vertebrae are of a horseshoe shape ; this may account 

 for the U-shaped hypural of the penultimate vertebra, which is so 

 general in the family. 



General remarks on the Ajjodes. 



There is a striking iiniformity in the structure of the caudal 

 fin in this sub-order, due possibly to similar habits. In spite of 

 observations, as early as Huxley's in 1859, to the contrary, writers 

 repeatedly refer to the Eels as possessing diphycercal or protocercal 

 tails. It cannot be too greatly insisted on, that whenever an 

 upturned urostyle is present, either in the adult or the larva, a 

 tail is not protocercal or diphycercal. Externally, of couise, the 

 Eels, with their undiflerentiated median fin system, do present a 

 primitive protocercal condition, but dissection proves the terminal 

 endoskeletal supports to be vexy unsymmetrical ; the broad 

 hypural bones have no corresponding dorsal homologues. The 

 caudal fin of the Apodes complies in all its essentials with 

 the definition of homocei'cy, which will be discussed later ; it 

 cannot be called anything but an advanced form of homocercal 

 caudal. 



While dealing with the Apodes, it is interesting to note the 

 references to the ancesti'al forms. The fossil Urenchelj/s fi-om the 

 Chalk beds has a homocercal tail — pi-esunrably an externally recog- 

 nizable homocercal tail — and probably, therefore, much less 

 specialized a form than the advanced form the modern Apodes 

 have been shown to possess. Thus all that has happened is a 

 steady process of specialization, exactly as is to be expected. 



