122 LOWER VERTEBRATES. 



In regard to these different groups, there is no doubt that each represents one or 

 more diverging lines of descent, and on the otlier hand that they are all more or less 

 intertangled through the presence of intermediate forms. The reasons for placing all 

 these forms in one order, as well as the general characters of that order, are thus indi- 

 cated by Professor Gill : " Among the most generalized of the typical fishes, and 

 which have been by common consent regarded as most nearly allied to the Ganoids, are 

 the physostomous Teleocephals, best known under the forms of the Cyprinoids, Clu- 

 peoids, and Salmonoids. With these the pikes, Scomberescoids, and perches, and, in 

 fact, all those forms most familiar to men at large, numerous as they are, appear to agree 

 in all material respects as to skeletal peculiarities and the character of the brain. ... It 

 may be said that a general description of the skull and shoulder girdle of a cod, a 

 perch, a mullet, a pike, a salmon, or an electric eel, would almost equally well apply to 

 the one as to the other, or any other teleostean fish, so far as the simple numher and 

 essential connection of the bones are concerned. The frontal bones may be single or 

 double, the anterior sphenoid may be present or absent, the palatine and pterygoid 

 bones may be distinct, or, as in the electric eel, in part fused together, the scapular 

 arch may be attached by one or two processes to the skull, a mesocoracoid may or may 

 not be persistent, and even the paraglenal bones may be quasi-cartilaginous ; but the 

 agreement in other respects is so close in contrast with the rej)resentatives of other 

 orders that the exigencies of classification seem to be best met by the union of all such 

 in one order. In all, the deviations in the skull are comparatively slight, and the 

 scapular arch is composed of a post-temporal and postero-temporal, the latter connect- 

 ing with the proscapula, while the paraglenal or coracoid is differentiated into at least 

 a hypercoracoid and a hypocoracoid, the latter two bearing the actinosts, which are 

 generally four or (rarely) five in number. With the iDostero-temporal or jjroscapula is 

 connected a ' post-clavicle,' from which is generally developed a second distal bone, 

 and sometimes (in Clupeidae) several. The brain, heart, and vascular system gene- 

 rally, and hyo-branchial apparatus are fundamentally similar, but exhibit (especially 

 the last) minor modifications that indicate narrower differences, and that may be 

 used in the distinction of inferior groups. For all the forms possessing the common 

 characters alluded to may be retained the ordinal name Teleocej)hali. 



" If a typical physostome fish (e. g. Clupeid) and a specialized j^hysoclist form (e. g. 

 Perca, Slennius) ai'c contrasted, the differences certainly appear to be considerable, 

 and are exhibited (1) in the jiresence or absence of a ductus pneumaticus; (2) the 

 position of ventrals, abdominal or anterior ; (3) the presence or absence of a meso- 

 coracoid ; (4) the junction of the parietals, or their separation by the intervention of a 

 supra-occipital ; (5) the presence of articulated branching rays, or their representation 

 by spines; (6) the low or comparatively high insertion of the pectoral fins; and (7) 

 the course of the lateral line, whether decurved in the direction of the abdomen, or 

 curved in the direction of the back. 



" But distinct as the forms appear to be when contrasted, numerous forms inter- 

 vene in which the characters successively disappear, or are combined in different ways, 

 and the most esteemed differential characters (presence or absence of the ductus 

 pneumaticus) are found in forms on the one hand so blosely related (Cyprinodontidos 

 vs. Synentognathi), and on the other hand so much differing from the next or adjoin- 

 ing forms, that the demands of classification appear to be best met by their union in 

 one order." 



In this view, groups representing several diverging tendencies or lines of descent 



