CLASSIFICATION 45 



not only of relatively huge size, but the gape is capable of considerable expansion. In 

 a closely related form, Pelecanichlhys, the mouth is even larger, the lower jaw projects 

 well beyond the extremity of the snout, and the mandibular membranes are so 

 voluminous as to form a distinct gular pouch. All these fishes are piscivorous, and 

 the wide mouth, expanding gape and distensible stomach enable them to seize and 

 devour the large fishes that have been found in their .stomachs. The teeth in the 

 jaws are small and curved, those of the mandibles being depressible inwards, and 

 the gill-rakers are quite rudimentary. 



The form and arrangement of the teeth, although closely linked with the nature 

 of the food, provide valuable generic and specific characters. The large, symmetrical 

 mouth and strong teeth of the Halibut or the Turbot, which are in the habit of leaving 

 the bottom and going in active pursuit of other fishes, are well adapted for such feeding 

 habits. Similarly, the smaller asymmetrical mouth of the Plaice or Flounder, with 

 the obtuse or incisor-like teeth developed mainly on the blind side, is correlated with 

 the habit of feeding on small invertebrates or fishes, which live on the sea floor and are 

 seized from above with the lower side of the jaws. Comparison of the normal diet of 

 the Plaice with that of the Flounder reveals the fact that the former includes a much 

 greater percentage of molluscs in its food ; the incisor-like teeth in the jaws and the 

 molariform teeth of the lower pharyngeals in the I-'laice are better fitted to deal with 

 food of this type than are the obtusely conical teeth of the Flounder. 



The form of the lower pharyngeals and their teeth, and the form and number of 

 the rakers on the gill-arches are likewise associated with the nature of the food, but 

 provide good specific or even generic characters in certain groups of Flatfishes. In 

 all the Bothidae and many of the Pleuronectidse the lower pharyngeals have the form 

 of a pair of narrow, rod-like bones, usually armed with one, two or more rows of 

 sharply pointed teeth. In some of the Pleuronectidae, however, and especially in 

 the more specialised Pleuronectinse with asymmetrical jaws and dentition, these 

 bones are broader, with their inner edges angular, and are united to a greater or lesser 

 extent to form a triangular plate, which is armed with series of conical, rounded or 

 molariform teeth. Starting with a form like the Dab (Limanda), in which the lower 

 pharyngeals are narrow, separate, and each armed with two series of conical teeth, it 

 is possible to trace a complete series of stages, culminating in a solid triangular plate 

 formed by the union of two broad pharyngeals, covered with a mosaic of molariform 

 teeth, such as is found in species of Liopsetta and Platichihys. In the Plaice (Pleuro- 

 nectes) the lower pharyngeals are firmly united, but are not so broad as in the genera 

 just mentioned, and the round molar-like teeth retain traces of the primitive biserial 

 arrangement found in Limanda. 



In addition to the position of the origin of the dorsal fin already mentioned, both 

 median and paired fins provide a number of taxonomic characters of varying impor- 

 tance. Chief among these are the branched or unbranched character of the individual 

 rays, the presence or absence of scales along the rays, their height or length, whether 

 free or united by membrane, the development of a scaly sheath at the base of the dorsal 

 and anal fins, the length of the pectoral fin, the absence of the pectoral or pelvic of the 

 blind side, the union of the pelvic with the anal, and the shape of the caudal. The 

 number of rays in the dorsal and anal fins, an important specific character, is dealt 

 with below. 



The presence or absence of an " anal spine " was originally considered to be a 

 feature of some importance, and one author has recently employed this as a generic 

 character (see p. 89). This so-called .spine is the lower extremity of a curved bone, 

 generally of considerable size, which is referred to throughout this work as the first 

 interhaemal spine. It forms the posterior boundary of the abdominal cavity, curving 

 downwards and forwards and ending in a point just behind the anus. Above it fits 

 into a deep groove on the anterior face of the hsmal spine of the first caudal vertebra 

 (Fig. 3). Both Kyle (iqoob) and Cole and Johnstone (igo2) have pointed out that 

 the projection or otherwise of this spine depends to a large extent on the state of 

 preservation of the specimen, and they regard its projection through the thin skin 



