316 



ICHTHYOLOGY. 



SUB-FAMILT IV. NEROPHIN.E. 



The eggs distributed in rows on the breast and belly of the males, 

 not covered by membrane. 



Genus I. Microphis, Kaup. Body heptangular, composed of 

 very rough rings, and of equal thickness to the setting on of the 

 elongated tail. Five fins. Two species. 



Gencs II. yEROPHis. Kafin. Neither pectoral nor anal fin. 

 Body cylindrical without distinct edges. Nine species. 



GANOIDS. 



Miiller remarks that the difference between Cycloid and 

 Ctenoid scales is slight, and its systematic application is 

 consequently confined to narrow limits ; but that the case is 

 very different with regard to Ganoid scales. These are 

 bony, generally of a rhombic form or quadrangular, seldom 

 rounded in outline or imbricated in position, and their sur- 

 face is always smooth and coated with a layer of enamel ; 

 they are generally arranged in oblique rows, those in each 

 row being usually united to one another by a hinge-like 

 prolongation of the anterior angle. The Ganoid scales are 

 imbedded, like ordinary scales, in depressions of the skin ; 

 but the epidermis is extremely thin, and adheres closely, so 

 as to appear wanting. Tliis is the case in Poli/pterus, but 

 in Lepidosteus the epidermal covering is more evident. 

 The living Ganoids have completely bony skeletons, but in 

 the fossil ones many have had skeletons soft and cartila- 

 ginous like those of the Sturgeons. Miiller sums up the 

 Ganoid character in the following words : — Fishes provided 

 with either tabular and angular or round enamelled scales, or 

 with bony plates, or a perfectly naked skin. Their fins 

 frequently, hnV not always covered on their anterior border 

 with a single or double row of spiny plates or laminae 

 {fulcra) ; their caudal fin sometimes involving in its upper 

 fold the extremity of the vertebral column, which may con- 

 tinue to the end of that fold ; their nasal apertures double ; 

 their gills free and lying in an operc\ilated cavity, as in tlie 

 osseous fishes. Several have an accessory organ in form of 

 an opercular gill, which is different from a pseudobranchia, 

 and may occur at the same time with it ; many have also 

 blowing holes like the P/affios/o?ni. They have several 

 valves in the arterial trunk, like the latter ; their ova are 

 conveyed from the abdominal cavity by tubes ; their optic 

 nerves do not decussate, but merely cohere laterally ; their 

 intestine often contains a spiral valve, like the Plagiostomi ; 

 they have a swimming bladder with an air-tube, like many 

 osseous fishes ; their skeleton is either bony or partly car- 

 tilaginous ; and their ventral fins are abdominal. If, how- 

 ever, reference be made to the absolute characters only 

 which are never absent, the Ganoids are /ishes with vu- 

 merons valves in the arterial trunk, no decussation of the 

 opttc 7ierves, free gills and opercula., and with abdominal 

 ventral fins. 



Agassiz included the Sclerodermi, Gymnodontes, Lorica- 

 rini, SiluridtF, and Lophobranchii among the Ganoids ; but, 

 according to Miiller, all these groups belong to the ordinary 

 osseous series, and have only two valves in the arterial 

 trunk. The existence of an accessory opercular gill is a 

 character of the Ganoids and Sturgeons, and does not oc- 

 cur in other osseous fishes. Miiller further observes that 

 the Ganoids resemble the Selachii in having a thymus 

 gland. This organ was described by Dr Simon as it exists 

 in the Sturgeons, and it is found in the same position in 

 Poh/pterus and Lepidosteus ; that is, between the anterior 

 basibranchials and the sternohyoid muscles. It was con- 

 sidered by Retzius to be a subhngual salivary gland, but it 

 is a vasoganglion ; and Professor Owen is of opinion that 

 Dr Simon's view of its homology with the thvraus gland is 

 more in accordance with its nature ; but he also thinks that 

 Dr Simon lias described as a thymus gland in various 

 osseous fishes the same parts which Miiller has called pseudo- 

 branchiae in those fishes. 



Lopho- 

 branchs. 



Characters distinguishing the Polypteri from Lepidostei Classifies' 

 exist in the solitary arrangement of the rays of the vertical t'on — 

 fins, the want of accessory gills, and the incompleteness of 

 the fourth gill, which has only one layer of leaflets, and 

 wants also the slit behind it. There is only one pancreatic 

 ca;cum, whereas the Lepidostei have many ; both have 

 muscular coats to the air-bladder, but in the Lepidostei 

 that viscus is cellular, and the muscular fibres are in bundles 

 between the folds of the bladder. The air-tube or trachea 

 in the Lepidostei is wide, and opens through the dorsal wall 

 of the oesophagus, its orifice bemg regulated by a sphincter 

 muscle. But the Polijpteri differ from all other known fishes 

 in the air-duct entering the beginning of the oesophagus or 

 gullet on its ventral side. The arteries of the swim- 

 bladder of the Lepidostei arise in great numbers from the 

 aorta, and the veins empty themselves into the two sub- 

 vertebral veins. In this case, therefore, the cellular swim- 

 bladder cannot be considered as a lung. In the Polypteri 

 the artery of each of the two air-bladders is formed by the 

 union of the bloodvessels coming from the last gill, and 

 therefore carrying blood already oxygenized. The Lepi- 

 dostei have sacciform ovaries, with oviducts issuing from 

 the middles of the sacs. These fish are by no means rare 

 in the North American waters, and a few years ago the re- 

 searches of Agassiz had raised the ntimber described to ten. 

 It is probable that since then more have been discovered ; 

 as he was of opinion, from what he had then observed, that 

 every separate river, basin, or watershed,. had its peculiar 

 species. The Lepidostei frequent shallow, reedy, or grassy 

 places, and bask in the sun like Pikes. 



Polypterus is an African genus. Dr Baikie, in his late 

 ascent of the Tchadda, obtained specimens of what he thinks 

 will prove to be a new species, but he has not yet been able 

 to compare it with the one that exists in the Senegal. The 

 thymus gland is double in Polypterus ; and the spiral in- 

 testine valve, which in Lepidosteus is but slightly developed, 

 attains its maximum size in this genus, extending upwards 

 to the entrance of the bile-duct. 



Amia is a genus of American fishes, existing in the fresh 

 waters of almost all parts of the United States, but chiefly 

 abounding in the muddy streams and lakes of Georgia and 

 Florida. At New Orleans the Amia are named "Mud-fish." 

 M. Valenciennes describes the genus among the Malacop- 

 terygians, and says that it exhibits numerous affinities with 

 other families of that order, agreeing with the CyprinidcB in 

 the simplicity of the intestinal canal and want of pyloric caeca, 

 though no Cyprinoid resembles it in having a spiral valve 

 in the intestine. Like Elops and Megalops, it possesses a 

 sublingual buckler. By the large size of its suborbitars, it 

 is allied to Erythrinus ; in its dentition, and in the structure 

 of its ovaries, it has marked connections with the Salmonoid 

 fishes ; lastly, in its general aspect, the length of the dorsal, 

 the form of the scales, and in the length and flexibility of 

 the body, it evinces close similarity to Ophicephalus. These 

 remarks of M. Valenciennes are enough to show that 

 ichthyologists were at a loss to find the proper place of this 

 genus when Dr Vogt, in 1845, discovered it to be a Ganoid, 

 for he found in its arterial trunk two obliq\ie rows of five 

 or six valves each, and tlie arterial trunk itself covered with 

 a well-defined layer of muscular fibres, as in other Ganoids. 

 It has, moreover, a spiral valve of several turns in the in- 

 testine, though it does not ascend so high up as in Polyp- 

 terus. But the scales of Amia are not Ganoid, and have 

 not the slightest resemblance to those of tliat genus or of 

 Lepisosteus ; and from this Muller infers that the structure 

 of the scales is not to be trusted as an ordinal character. 

 The scales oiAmia are not osseous plates ; they are flexible 

 and rounded, and destitute of enamel. Similar scales belong 

 to the fossil Mrgahirus and L^eptolepis, which are considered 

 by Agassiz to be Ganoids. In habit Amia resembles the 

 osseous fishes rather than the Ganoids. As Ainia agrees 



