88 SPECIAL CONVERGENCE 



genera of a family, to the families of an order, 

 to the orders of a class, to the classes of a 

 phylum and to the phyla of the animal kingdom. 

 One of the best examples of special family 

 convergence is afforded by the pectoral fins of 

 flying fishes. Semper (op. cit.) dealt with the 

 organs of flight in Vertebrata, and figured a 

 flying "herring," Exocostus, belonging to the 

 Teleostean family Scombresocidae, in juxta- 

 position with a bat ; but he did not refer to 

 the case of the flying gurnard, Dactylopterus, 

 which belongs to another family, the Triglidse 

 (sometimes called Cottidse and Cataphracti). 

 Both of these genera occur in the Mediter- 

 ranean as well as in the Indian Ocean, and 

 are totally different from each other, not only 

 in systematic position but in external form. 

 Dactylopterus has a broad, depressed head, 

 armed with powerful spines, on account of 

 which the head is commonly fractured by a 

 blow when the fish is caught by native fisher- 

 men off the coast of Ceylon ; the scales are 

 hard, keeled scutes ; the tail - fin is truncated, 

 the abdomen flattened, and the body coloured 

 red. Exoccetus has a normal herring-like head 

 and body, unarmed, with smooth scales, a deeply- 

 forked tail-fin, a convex abdomen, and a silvery 

 ground colour. Both kinds of flying-fishes owe 

 their limited power of flight above the surface 

 of the sea to the secondary elongation and 



