564! peof. t. "w. beidge ok" the mesial 



Peecid^. 



JPerca fluviatilis. 



Except for tVie absence of mesial segraents, the fins and fin- 

 supports of this species have a fairly close resemblance to tbose 

 of Soloeentrum. 



Anterior Dorsal fin. — This fin consists of fifteen spinose rays 

 supported by a like number of radial elements. All the radial 

 elements are bisegmental except the last three, which have 

 proximal segments only. Most of the proximal segments have 

 well marked postero-superior processes which appear to take the 

 place of the missing mesial segments. As in Solocentrum, all 

 the distal segments are provided with hook-like processes. The 

 distal segment of the first element seems, however, to have fused 

 with the proximal segment, a groove at its base alone indicating 

 its original distinctness. 



A median dorsal bony groove for the reception of tbe deflected 

 spines is present in Perca, but the successive sections are formed 

 by lateral wings developed from the postero-superior processes 

 of the proximal segments, in conjunction with those contributed 

 by the distal segments. 



The first spine has a " chain-link " articulation with the 

 distal end of the first proximal segment. The second, a similar 

 articulation with the corresponding distal segment, and, in 

 addition, a basal articulation with a transversely elongated 

 articular surface on the distal extremity of the second proximal 

 segment. The remaining spines have precisely similar arti- 

 culations, two successive elements contributing to the support 

 of each spine. The last three radial elements being without 

 distal segments, it follows that the last and penultimate spines, 

 which are very feebly developed, and rightly belong to the 

 thirteenth and fourteenth elements, are supported solely by the 

 fourteenth and fifteenth proximal segments respectively. The 

 first two of these spines have simple cleft basal ends, without 

 articular surfaces, and merely clip the dorsal extremities of their 

 fin-supports ; the third is a simple undivided ray, and is supported 

 by fitting into a cleft in the distal extremity of the last proximal 

 segment. 



Anterior to the first ray-bearing radial element there is a 

 vestigial proximal segment which is probably the normal fin- 

 support to the first spine. 



