Classifcation of the Order Discocepliuli. 



G35 



extends forward on the upper surface of the head. The 

 spines are shortened, depressed backwards, and expanded 

 transversely to form pairs of broad flat laminae which are 

 denticulated posteriorly ; in the middle line posterior pro- 

 jections of each pair of lamiu?e are connected by ligament, 

 and a low fin-membrane joins them to the next pair. This 

 is the structure in the Echeneididse ; in the Eocene Opistho- 

 myzonidse the laminae were not nearly so broad and the 

 median projection seems to have been stronger and un- 

 divided. 



liemora ipmoru. Segments of disc from above (A) and below (B). 

 /, fin-ray ; r, radial ; b, basal. 



The small lateral expansions of the radials seen in most 

 Percoids have become large overlapping laminae in the 

 Echeneididse ; the basalia are nearly normal, except that 

 they are directed very obliquely backwards. 



It is difficult to imagine the initial stages of this extra- 

 ordinary modification, but it seems not impossible that some 

 pelagic Percoid with the habit of associating with sharks, 

 like the pilot-fish (Naucrates), should have found that the 

 spinous dorsal fin, when depressed in its groove, could be 

 used more or less effectively as an adhesive organ, and 

 shoukl have acquired the habit, when swimming close to its 

 protector, of fastening on. 



The other fins are very similar to those of the Pomatomidte, 

 Carangidae, Rhachicentridae, &c., in structure and position, 

 and the Discocephali may well have evolved from Percoids 

 of this type. 



The most important characters of the Discocephali, in 

 addition to the peculiar structure of the dorsal fin, are the 

 following : — 



Mouth not protractile, with the slender maxillary firmly 

 adherent to the praemaxillary ; dentary attached to articulare 



