J. K. Tharhr)' — Median and Paired Fins. '285 



In general the ^<l^u•tul•e of tlie medi.-ui tin resembles wliat we have 

 seen in Petromyzon, but there are important differences. The rays 

 are of hyaline cartilage and they do not usually reach down to the 

 ridge-pole of the myelonal canal. In EKlamla, for example, in tlie 

 smaller of the specimens %ured, the rays approach within a centime- 

 ter and a half at the oiad extremity of the first dorsal fin, but are 

 three and a half centimeters distant at the other extremity. In 

 Si/ualus {Ae(()ifh)as) they come closer. 



This ridge-pole consists of a cord of rather peculiar white lono-i- 

 tudinal fibers, constituting now a '' liyauieatuin longltitdinale.'''' It 

 appears to me undoubtedly homologous with the fatty-fibrous body 

 in Petromyzon. The cartilaginous arches unite under, and do not 

 extend around over it, though they clasp it somewhat. 



The rays are segmented, usually twice. Dichotomy is rare if not 

 altogether absent. Concrescence of adjacent rays is by no means 

 uncommon. The reduction of rays in size is exhibited in all deo-rees. 



Calcification presents itself in a thin superficial layer on each side 

 of the somewhat flattened ray, but it fails on the edges, i. e. as we 

 come close to the median plane. 



The muscles of the fin, as in Petromyzon, are in total independ- 

 ence of the large masses of segmented lateral muscles, but they are 

 in more definite relation with the skeleton of the fin. This is accu- 

 rately represented in PI. LIX, fig. 66, though that is a s-;ction of a 

 pectoral and not of a median fin. We see that each ray has on each 

 side a special muscle, separated from its fellows by the fibrous sheet 

 which runs from between the rays to the integument. Each little 

 muscle developes in its median line a flat tendon, which, parallel to 

 the surface of the fin, inserts itself in the fascia covering the extremi- 

 ties of the fin-rays and the proximal ends of the well known horny 

 fibers, which here supplement the primordial skeleton, as the second- 

 ary fin-rays of Ganoids and Teleosts do. 



The relation between the niimber of fin-rays and that of the 

 vertebra? opposite to them is similar to what we ha^e seen in the 

 lower forms. In the Nictitantes, for example, there are on the average 

 about '2-5 rays to one vertebra. But there is considerable variation, 

 even in individuals of the same species. The extreme numbers, so far 

 as I have observed, are 3 '5 in an anal of Sphyrna, and 2 in a first 

 dorsal of Eulamia. 



We turn now to the more minute examination of several species.. 



