Fishes of the Western North Atlantic 7 



swim bladder is not a feature of much direct service to the systematist, for groups 

 that differ widely in other respects may agree in lacking the swim bladder, and 

 vice versa.21 



In some families, even where most of the genera have a swim bladder, others do 

 not. Familiar examples of such families are the Blenniidae (blennies), the Polynemidae 

 (threadfins), and the Scombridae (mackerels); in the last group the chub mackerel 

 {Pneumatophorus) has a swim bladder while the common mackerel of the North Atlantic 

 {Scomber) has none (Starks, g2\ 223). 



It has long been emphasized that, while the gill folds of elasmobranchs and chi- 

 maeroids are attached to the interbranchial septa outward nearly to their tips, those of 

 the higher groups of bony fishes are free from the septa for most of their lengths. These 

 extremes, however, are bridged on the one hand by the Dipnoi, where the gills reach 

 only slightly beyond the septa, and on the other hand by the polypteroids, acipenserids 

 (sturgeons), and lepisosteids (gars), where they extend only a little farther outward. 

 Furthermore, most of the bony fishes lack functional respiratory folds on the anterior 

 (hyoidean) wall of the first gill pouch, such as are characteristic of elasmobranchs and 

 chimaeroids; but the acipenserids and lepisosteids are exceptions to this rule, as is the 

 living coelacanth Latimeria (Millot and Anthony, 31: 2584). 



The ventral or pleural ribs between the adjoining surfaces of the peritoneum and 

 body wall are diagnostic for such of those bony fishes as have them, for the ribs of 

 the living cartilaginous fishes^^ that have any lie in the horizontal septum that separates 

 the musculature of the trunk into an upper and lower division.^' Many bony fishes, 

 however, lack ribs of any sort: the lophobranchs {26: 28—30), for example, the gi- 

 ganturoids (yo: 57), the stylophorids (80: 21), and some trachypteroids (genus 

 Agrostichthyes) (information from Vladimir Walters). The tetraodontids (swellfishes), 

 the diodontids (porcupinefishes), and the ostraciodontids (trunkfishes), among the 

 plectognaths, lack ribs (j6: 8, 10; 41: 325, pis. 15, 16), while their relatives, 

 the Balistidae (triggerfishes) and Aleuteridae (filefishes), have ventral ribs.-* It is also 

 evident from a glance at the skeleton of Lophius (anglerfish) that it is ribless, as are 

 the Ogcocephalidae (batfishes), by personal observation, and Rhynchoceratias (60: 13), 

 among pediculate fishes. 



So-called dorsal ribs are not peculiar to the elasmobranchs alone, for the po- 

 lypteroids, salmonids (salmons), some clupeids (herrings) (26: 209), Esocoidea (pikes) 

 (4: 242, 429), and balistids (triggerfishes) among the plectognaths (41: 304) have 

 both ventral ribs and ribs that occupy the dorsal position. However, it appears that 



21. N.B.Marshall contributes the information (personal communication) that the major groups of bathypelagic 

 fishes, such as stomiatoids, deep-sea salmonoids, and Miripinnati, can be characterized by the structure of the 

 swim bladder. 



22. The chimaeroids and some of the rays are ribless. 



23. But these ribs are considered by Emelianov (26: 244) to be homologous in their development with the pleural 

 ribs of bony fishes. 



24. Regan (66: 285, 286, fig. 56) interpreted the rib-like bones of the Balistidae and Triacanthidae as intermuscular 

 in nature and "not bordering the abdominal cavity." But it seems clear from KashkarofF's account {41: 303, 

 325, pis. 12-14) ^^'^ from our own dissections that they are true ventral ribs and do embrace the body cavity. 



