=Order Heteromi.=—The order of _Heteromi_ (ἑτερός, different; ὤμος, shoulder), or spiny eels, may be here noticed for want of a better place, as its affinities are very uncertain. Some writers have regarded it as allied to the eels; some have placed it among the Ganoids. Others have found affinities with the sticklebacks, and still others with the singular fresh-water fishes called _Mastacembelus_. The _Heteromi_ agree with the eels, as well as with _Mastacembelus_, in having the scapular arch separate from the cranium. Unlike all the true eels, most of the species have true dorsal and anal spines, as in the _Percesoces_ and _Hemibranchii_. The ventral fins, when present, are abdominal and each with several spines in front, a character not found among the _Acanthopteri_. There is no mesocoracoid. The air-bladder has a duct, and the coracoids, much as in the _Xenomi_, are reduced to a single lamellar imperforate plate. The two groups have little else in common, however, and this trait is possibly primitive in both cases, more likely to have arisen through independent degeneration. The separation of the shoulder-girdle doubtless indicates no affinity with the eels, as the bones of the jaws are quite normal. Two families are known, both from the deep sea, besides an extinct family in which spines are not developed. The _Notacanthidæ_ are elongate, compressed, ending in a band-shaped, tapering tail; the back has numerous free spines and few or no soft rays, and the mouth is normal, provided with teeth. The species of _Notacanthus_ are few and scantily preserved. Those of _Macdonaldia_ are more abundant. _Macdonaldia challengeri_ is from the North Pacific, being once taken off Tokio. The extinct family of _Protonotacanthidæ_ differs in the total absence of dorsal spines and fin-rays; the single species, _Pronotocanthus sahel-almæ_, originally described as a primitive eel, occurs in the Cretaceous of Mount Lebanon. The _Lipogenyidæ_ have a round, sucker-like mouth, with imperfect lower jaw, but are otherwise similar. _Lipogenys gilli_ was dredged in the Gulf Stream. [Illustration: FIG. 118.—_Notacanthus phasganorus_ Goode & Bean. Grand Banks. ] Dr. Boulenger has recently extended the group of _Heteromi_ by the addition of the _Dercetidæ_, _Halosauridæ_ (_Lyopomi_), and the _Fierasferidæ_. We can hardly suppose that all these forms are really allied to _Notacanthus_. CHAPTER VIII SERIES OSTARIOPHYSI