l'l,i:( TOCiNA'l'ES. 



619 



linve iiria'iiially started tV((iii the (loriual s\st(_'iu, as tor 

 i'\aiiii)k' tlie clavicles, here in many cases iiave the 

 character nf Ganoid scaly or scutate growths in the 

 sl<in. This l'airi\ iironiiiient touch of resemblance to 

 the (lanoids has induced scientists long to retain the 

 majority of the t'oilowing I'Insoclvsts among tiie lowest, 

 least modern Teleosts, though the characters I)y which 

 lioth the Plectognates and the LophnJirnitchU have been 

 distinguished ever since the time of Cuviku, are evi- 

 dently later modifications (higher metamorphosic degrees) 

 of the 'i'cieostean ty])e. 



riie Plectognates compose an order fairly rich in 

 forms, varying both in the form of the body and, still 

 more, in th(! texture of its covering. Some of them, 

 most of tJK^ Balistoids, are of a fairly regular piscine 

 form, others of a polygonal form, which has given them 

 the name of Coffer-tishes, others again, the Gymnodonts, 

 of an elongated saccate or globular foi'm. Some have 



bilid at the top and united to tiie etiuuoid hone by 

 ligaments. But in other cases, as in tlie geiuis 'I'ctrodon, 

 it resembles the pterygoid bone in form and is firmly 

 united to the lower surface of the ethmoid bone. Both 

 the palatine bones and the vomer are toothless; and the 

 piiai-yngeals, which are tree, may, it is true, })e armed 

 with teeth, as in the Balistoids and Tetrodonts, but may 

 also be destitute of teeth, as in the Coft'er-fishes. The 

 shoulder-girdle is generally well-developecL The post- 

 temporal bone is indeed insignificant, lieing confluent 

 with the mastoid b(jiie, and the supraclavicular bone 

 shows scarcely anything unusual in its development; but 

 the clavicular bone proper, though sometimes of ordi- 

 narj' form, may extend so far in\vards in some of the 

 Plectognates, as in the Coffer-fisiies, tiiat tlie bone of 

 one side is contiguous to that of the other throughout 

 the depth of the median ])lane of the abdominal cavity, 

 thus forming a kind of diaphragm of bone with only a 



hard, granulated scales, densely imiiricated, juxtaposed [ round, more or less tuliular hole in the middle for the 



to each other like a mosaic, or firmly united to a ca- 

 rapace that may cover the greater part of the body, 

 oliiers small, spiny scales or, in their stead, loose, mo- 

 bile spines, sometimes of considerable size, which are 

 erected in self-defence. All the Plectognates have a 

 comparatively small, but well-armed mouth, with strong, 

 hut few teeth, the strength of which is increased by 

 tlie firm support afforded them by the coalescence and 

 reduction of the component ]iarts of the jaws. The 

 short jaw bones form the tip of the facial part of the 

 head, which part is generallj' considerably elongated 

 and also deejjened. This elongation chiefly affects the 

 ethmoid bone, the extent of which on the upper sui'face 

 of the cranium may rival that of the frontal l)ones, and 

 the parasphenoid bone (the sphenoid bone of fishes), 

 hut is accompanied by the elongation of the preoper- 

 culum and interoperculum". The last-mentioned bone 

 has a narrow, terete shaft, which extends from the hind 

 inferior angle of the lower jaw along the inside of the 

 lower anterior prong of the preoperculutn, just as the 

 latter extends hack from the otiter surface of the pro- 

 cess with which the lower jaw articulates, on the qua- 

 drate l)one. The deepening of the facial part depends 

 chiefly on the great vertical extension of the parasphe- 

 noid bone and the pterygoid arch. Sometimes, it is 

 true, as in the Balistoids, the palatine bone on each side 

 may be a small, terete, and more or less mobile bone, 



passage of the cesophagus. The shoulder-girdle of the 

 Coft'er-fishes is still further extended, below the coracoid 

 bones, by a pair of large 'interclavicles,' of which in the 

 prciceding fishes we have found scarcely a trace, but 

 which we shall again meet in the Sticklebacks in the form 

 of Ganoid plates. In BaUstes the clavicle extends so 

 high u]) that with its ujjper extremity it touches the 

 cranium, at the styloid bone, just within and under the 

 articular surface of the supraclavicular bone. 



In Aktedi the Plectognates formed the framework 

 of the order which he called BrancMostegi (i. e. ivitk 

 hidden gills, a reference to the narrow gill-openings just 

 above or in front of the insertions of the pectoral fins); 

 and the principal character of the order, according to 

 its foitnder, lay in the absence of special branchiostegal 

 membranes. In general, however, the exact opposite is 

 the case; the opercular apparatus is by no means im- 

 perfect in these fishes. The operculum and suboper- 

 culum are small, it is true, in the Balistoids and Coffer- 

 fishes, but of normal size in the Tetrodonts. The ad- 

 vanced development of the preoperculum has been 

 mentioned above. The hyoid arches, to which the 

 branchiostegal membranes with their 4'' — 6 ra\s are 

 attached, have gained in breadth what they have lost 

 in length; and in most cases one of the rays in each 

 membrane, usually the innermost, is considerably stronger 

 than the others, sometimes, as in Tefrodon, several times 



This elongation and deejicning is most marked in the Scleroderins (see l)clow); in the Gymnodonts it is less prominent. 

 Sometimes 3, accordina: to Blkekeu. 



