PISCES. 



961 



which does not exist at all upon the opposite 

 side of the head, while anteriorly the anterior 

 frontals (2) and the nasal bone (20) complete 

 this part of the face. An orbital cavity is thus 

 constructed upon the right or upper side of the 

 head of the I j leuronectida3, which suffices for the 

 lodgment of the two eyes, which thus take the only 

 position in which both could be made useful.* 



Another equally remarkable arrangement is 

 observable in the construction of the jaws of 

 the Pleuronectidee, which are in many genera 

 very unequally developed on the two sides of 

 the median line, only in this case the prepon- 

 derance of developement is just the reverse of 

 what exists in the orbital portion of the face, 

 for here the bones of the right or upper side are 

 small, while those of the left or inferior half 

 are of considerably greater size and strength. 

 Moreover, the former are but sparingly fur- 

 nished with teeth, while the latter support the 

 chief part of the dental apparatus ; so that by 

 this structure the mouth becomes twisted toward 

 the ground, and the teeth so disposed as to 

 work most effectually in that direction. 



In the Syngnathidae, Ostracions, and other 

 Fishes, where the exoskeleton is inordinately 

 developed, so as to form a suit of bony armour 

 in which the exterior of the body is completely 

 covered, the endoskeleton is proportionately 

 weak and imperfectly formed, many of the 



Fig. 494. 



bones remaining in a rudimentary condition. 

 This is well seen in the osteology of the Fly- 

 ing Hippocamp, (Pegasus draco, fig. 494,) 

 where the bones both of the head and trunk 

 seem to perform quite a secondary part as 

 contrasted with the dense tegumenlary frame- 

 work covering the body. The whole face seems 



" The above description of the structure of the 

 face in the Pleuronectidas is derived from a disarti- 

 culated skull of the Halibut, contained in the Mu- 

 seum of the Royal College of Surgeons in London. 



VOL. III. 



to be composed of a kind of snout (a), derived 

 from the external armour of the head, in which 

 the orbits (b) are excavated, the intermaxillary 

 bones (d) and the lower jaw (e) alone being 

 recognisable. The gill covers (A) belong to the 

 exoskeleton, while the hyoid apparatus (_/') and 

 branchial arches (g) are but very imperfectly 

 formed. The osseous zone that sustains the 

 pectoral fins consists of a single bone, which is 

 so consolidated with the tegumentary skeleton 

 as completely to separate the abdominal cavity 

 from the branchial chambers; and in the piece 

 m, which forms the basis of the fin itself, the 

 usual divisions are quite undiscernible. The 

 vertebrae, both of the back and tail, (, </,) are 

 reduced to mere bony rings, while the pelvic 

 circle, (p,) that supports the abdominal fins, (/,) 

 is, like the rest of the skeleton, firmly con- 

 nected with the external bony armour. 



In the construction of the anterior extremities 

 a few peculiarities may be specified. 



Certain genera, more especially the Salmo- 

 nidceand the Cyprinidte, have attached to the ra- 

 dius and ulna upon their inner side a third bone, 

 which by its anterior extremity is connected to 

 the anterior margin of the os humeri, thus 

 forming a kind of buttress to support the fin. 

 In the Sduridee there likewise exist three bones 

 in the fore-arm which at an early period be- 

 come consolidated to each other, probably on 

 account of the great strength requisite 

 in that race of Fishes to support the great 

 spinous ray of the pectoral fin. In the 

 MttroEnidee, where there are but two 

 bones, these are suspended to the arch of 

 the shoulder at the point of junction be- 

 tween the scapula and the humeral bone. 

 In species that have no pectoral fins the 

 radius and ulna do not exist. 



There still remains to be noticed a 

 long styliform bone generally composed 

 of two pieces (fig. 437, 49, 50, p. 827, 

 andy/g.493, 10), of which the upper piece 

 (49), more or less flattened in shape, is 

 suspended from the os humeri (48), to 

 the posterior and superior part of the inner 

 surface of which it is adherent. This 

 styliform bone runs backwards along the 

 side of the body behind the pectoral fin, 

 and is plunged to a greater or less extent 

 amongst the flesh.* Some anatomists 

 have regarded this process as the homo- 

 logue of the clavicle, but from the posi- 

 tion which itoccupies, running backwards, 

 it seems rather to represent the coracoid 

 bone, which is in this case lost among the 

 muscles on account of the want of a 

 sternum to which it might be articulated. 

 It sometimes happens that this bone unites with 

 that of the opposite side, and occasionally 

 is of such length and strength as to reach 

 backwards as far as the commencement of the 

 anal fin. A not less curious disposition of this 

 bone is observed in Batrachus, where the supe- 

 rior division extends upwards beyond the hu- 

 merus to be connected with the spinous process 

 of the first vertebra. 



* Cuv. Hist, des Poissons. 

 3 Q 



