THE ORDERS OF TELEOSTOMOUS FISHES 483 



The Halosauries, Thornbacks , etc. 



This order as defined by Boulenger embraces certain eel-like * 

 deep-sea fishes formerly assigned to the suborders Lyopomi 

 Gill (Halosauridae) and Heteromi Gill (Notacanthidae) , together 

 with the more recently discovered Lipogenyidae which are said 

 to bridge over the gap between these two groups. To these 

 Boulenger adds the marine Fierasferidas and the Cretaceous 

 Dercetidae. All these families retain archaic or Isospondylous 

 characters in the abdominal position of the many-rayed ventral 

 fins (when present), especially in the union of the broad parietals 

 along the median line which widely separates the supraoccipital 

 from the frontals. "They are all characterized," says Smith 

 Woodward, 1 "by a primitive cranium of the Jurassic type; but 

 they exhibit the new specialization by which the extending 

 premaxilla gradually excludes the maxilla from the upper border 

 of the mouth. Their elongated shape alone is indicative of high 

 specialization; but no intermediate forms are yet known to afford 

 a clue to their more normally shaped ancestors. ' ' On the other 

 hand, they parallel the Acanthopteroidei in the closure of the air 

 bladder, in the absence of the mesocoracoid arch, and in the 

 frequent appearance of spines in the fins. The pectoral arch is 

 suspended from the supraoccipital or the epiotic (as in the 

 Iniomi), the posttemporal is small and simple, or replaced by a 

 ligament. The group parallels the Macruridae among the Ana- 

 cant hini and many other eel-like forms in the loss of the homo- 

 cercal tail (which is, however, preserved in the Cretaceous Der- 

 cetidae) and its replacement by the hypocercal type (Appendix II) . 



The existing Halosaurus "cannot be clearly distinguished 

 from the Cretaceous Echidnocephalus ; while Notacanthus of the 

 present fauna only seems to differ from Protonotacanthus of the 

 Cretaceous period in the possession of dorsal spines and fin-rays. 

 The Dercetidae, on the other hand, are only known by fossils 

 from Cretaceous formations, in which they are widely distributed. 

 They are interesting as being the earliest type of fish in which 

 evidence of a distensible stomach has been observed. . . • . 



1 See Appendix I. 



2 Cat. Foss. Fishes, Brit. Mus., Part IV, 1901, p. viii. 



