28 SORTATION. 



Two dorsals 



First dorsal above interval between ventrals and anal 

 Anal long Pristiurus. 

 Anal moderate Scyllium. 



First dorsal above interval between pectorals and ventrals^ 

 Eye with nictitating membrane. 



Head hammer-shaped Zygana. 

 Head conical ; 



Pit at base of caudal Carcharias. 

 No pit at base of caudal ; 



Second dorsal almost as large as first 



Mustehts. 



Second dorsal small Galeus. 

 Eye without nictitating membrane 



Tail not keeled 



Upper lobe very long Alopecias. 



Tail keeled 



Teeth large and lanceolate Lamna, 

 Teeth small and conical Selache. 



At last we have reached the fishes with homocercal tails, and by 

 eliminating the two flying fishes (Exoccztus), in which the pectoral 

 fins are developed into organs of flight, and the lower lobe of the 

 tail seems to be lengthened for steering purposes, we are left with 

 the large group in which both lobes of the tail are similar in outline. 

 The dorsal fins will guide us through the crowd, but before going 

 straight ahead we can simplify matters to a great extent by clearing 

 the road of some of the eccentricities. 



To begin with, there is one fish in the British list which seems 

 to have had its tail broken off short and stuck on again in a hurry at 

 the wrong angle. This is the deal-fish, Trachypterus arcticus, with two 

 dorsals, a short aiid a very long one, and no anal. In the others 

 the tail is in a line with the backbone. 



Two of the genera have short, stout bodies, one of them, Tetrodon 

 (the globe-fish), having the gullet dilatable into a sphere dotted with 

 star-like spines ; the other, Batistes, having the scales developed into 

 a coat of armour. Of Batistes there are two species, the trigger-fish 

 (B. maculatus), which has spots on the sides and no plates behind the 

 gills, and the file-fish (B. capriscus), which has no spots on the sides 

 and two plates behind the gills. 



Four genera are of little breadth, but much depth. Of these the 

 opah (Lampris) is iridescent, and marked with rounded, silvery 

 patches ; the dory (Zeus) is grey, with a large round black blotcli 

 ringed with yellow, and has ragged filaments attached to the dorsal 

 spines ; the trumpet-fish (Centriscus) has a lengthy tubular snout, 

 not unlike the chanter of the bag-pipes; and that very queer fish, 

 Argyropelecus,\mov/n by only one specimen, is square in the fore 

 body, becoming suddenly narrow as if the hinder half of a slender 

 fish had been telescoped into it. 



Four more genera we can group together, as having the body 

 elongated and slender. Two of them are pipe-fishes, armoured with 



