160 



Stukionidans.- 



-FISHES.- 



-AciPENSEROIDS. 



only gonus is Orthagoriscus or Orthrugoriscus, of which 

 two species frequent our seas, viz., the Short and the 

 Oliloug Molebut {Orth. mola aud ohlongus). 



The Tetroijonts, forming a family or sub-family 

 of some ichthyologists, is equivalent to the genus Tcl- 

 riiodon of Limiaius and Cuvier. They have their 

 gorget-shaped jaws, in which the teeth are incorporated, 

 divided above and below by serrated, mesial sutures, 

 forming four dental masses, whence the name. The 

 skin is partially and variously armed by spines, which 

 are smaller than those of the Diodonts, and in an 

 electric species are wholly absent. Some genera 

 have indatiblo liodies ; others do not possess the 

 ])()wer of inlLition, and have a moderately elongated 

 furni. 



The genera are — Tetraodon; Physogaster (Miillcr); Chtlo- 

 nodon (id.) ; Cheilichthys (ij.) ; and Arothron (id.). 



Some of these groups have no olfactory sac, or oven 

 depression, the nerve of smell being expended on a 

 solid, imperforate barbel. 



In the Triodonts the mandible is undivided, as in 

 the Diodonts, while the upper jaw has a mesial suture 

 as in the Tetrodonts. An immense dewlap is sustained 

 in front by a very largo bone representing the pubics, 

 and resembling the same parts in some Ba'istids. The 

 fins are hke those of the Diodonts, and the asperities of 

 the body and dewlap resemble those of the Tetrodonts. 

 A single species from the Indian seas is called Triodon 

 l/nrsarius, and is described by Lesson and Gamut in 

 their ichthyology of the voyage of I)upcrrcy. 



GANOIDS. 



Typical g.inoid scales have generally a rhomboid or 

 quadrangidar outline, with a smooth, shining coating 

 of enamel. They are seldom rounded, and rarely 

 imbricated, being more commonly arranged in oblique 

 rows whose cages touch ; but each scale is secured to 

 the one that precedes it by a hinge-like process of its 

 anterior corner. Like ordinary scales, the ganoid plates 

 are developeil in follicles, but their epidermal coating 

 is so very thin and so closely adherent as to be nearly 

 imperceptible. Miillcr sums up his view of the ganoid 

 character in the following terms : — Fishes provided 

 with tabular and angular or roundish enamelled scales, 

 or with bony plates, or with a perfectly naked skin ; 

 having fins fi'equently, but not always, sustained in 

 front by a single or double row of spiny plates, techni- 

 cally named fulcra; and a caudal fin involving in its 

 upper fold the extremity of the vertebral colunm, which 

 may continue to the end of the fold. The free gills are 

 placed in an operculated cavity, like those of the true 



osseous fishes. Several Ganoids have an accessory 

 branchial organ in form of an opercidar gill, which 

 differs from a psoudobrancbia, and may co-exist with 

 it. Many have spout-holes [spiracula or blowing- 

 holes) like the Plagiostomes. In the arterial bulb or 

 stem there are several rows of valves. The eggs are 

 generated in ovisacs and discharged through oviducts. 

 No decussation (cliiasma) exists between the optic 

 lobes, but merely a lateral cohesion. The swim-blad- 

 ders have pneumatic tubes ; the skeleton is bony, or 

 partly cartilaginous; and the ventral fins are situated on 

 the abdomen. According to the researches of Kcilliker, 

 the skeletons of the Plectognatbs and Lojihubranchs 

 are destitute of bono corpuscles, but those of the other 

 orders enumerated as Ganoids by Agassiz contain these 

 organisms. All the above characters do not exist in 

 every Ganoid ; and the essential parts of structure are, 

 according to Miillcr, several roivs nf valves in the arterkd 

 bulb, free operculated gills, and abdominal ventral fins. 



Order IX.— S T U III ON I D A N S. 



Family I.— ACIPENSEROIDS. 



(Plate 14, fig. 72 ; Plato 15, figs. 73, 74.) 



This order containing but one family may bo charac- 

 terized as a single group of Ganoids, whether these be 

 termed with Agassiz a class of Vertebrals, or with 

 Miillcr an order of Fishes. The distinctive characters 

 of the Acipenseroids {Sturiones, Cuvier) are, according 

 to lleckel and Kner, an elongated body, protected and 

 made more or less pentagonal by five rows of bony 

 shields; a protractile, toothless mouth on the ventral 

 aspect ; a blowing-hole and a single operculated gill- 

 opening on each side, but no branchiostegals in the 

 membrane ; a cartilaginous internal skeleton ; a hetero- 

 ccrcal caudal, densely bordered with short bony rays in 

 the upper lobe ; and a long spiral valve in the great 



intestine. In the genus Scaj)Mrliynclius the tail is 

 depressed at the root of the caudal, which is not the 

 case in the members of the more populous genus, 

 Acipenser (Sturgeon) nor in that of Planirostra or Poli/- 

 odon. Of tliese forms the family and order consist.* 

 The species are numerous in the temperate parts of the 

 northern hemisphere, both in the Old and New Worlds, 

 but they do not enter the arctic regions, and arc 

 unknown in the South Seas. 



Among the ancients, says Plinj', the Acipenser was 

 considered to be the noblest of fishes ; but it had ceased 

 to be esteemed in his time, at which he expresses his 

 surprise, since it was rare. The Atlllus of the Po, 

 which is considered by modern ichthyologists to be of 

 the same genusj is stated by Pliny to be sluggish and 



* Dlnecffs and Proceros are two genera of tliis family, named 

 but not sulliciently characterized by Kafinesque-Smaltz. 



