1906.] 



SELACHIAN PISHES. 725 



sacs, each with a single external aperture *. Exoskeleton of 

 dermal denticles which are strncturally identical with the teeth ; 

 no membrane-bones t. Endoskeleton cartilaginous, the cartilage 

 often calcified. Gills supported by visceral arches aiad functional 

 throughout life ; no lungs J. Median and paired fins with horny- 

 dermal rays § and with endoskeletal supports in the form of 

 series of cartilaginous rods, the arrangement of which may be 

 variously modified. Vertebral column comprising the notochord 

 and its sheath, simple neural and hsemal arches, and intermuscvilar 

 elements, the so-called i-ibs ; no supra-neural or infra-hfemal 

 arches ||. In all the livmg forms, the males with intromittent 

 organs, the mixopterygia, which are appendages of the pelvic fins. 



* The Teleostomi have two nostrils on each side, except in certain specialised 

 Teleosts. It has been supposed that these are homologous with the external and 

 internal nares of higher Vertebrates, the Dipneusti, in which the posterior nostril is 

 included within the mouth, being cited as evidence of this, especially by those who 

 consider them to be transitional between Pishes and Batrachians. The development 

 of these parts in Rei^tilia, Aves, and Mammalia has also been considered to support 

 this position, the internal nares being the remnants of open grooves originally 

 connecting them with the olfactory pits. 



From this view I must dissent, regarding the external nares, whether single or 

 divided into two, as homologous throughout the Fishes and the higher Vertebrates, 

 and the internal nares as a new formation peculiar to the latter. In the Dipneusti 

 the absence of ]ir£emaxillaries and maxillaries permits the extension of the nasal sacs 

 to the palate ; but it appears to me that the posterior extension of the nasal sacs 

 external to functional prwrnaxillaries and maxillaries, so as to carry the posterior 

 nostril into the mouth, is quite impossible. In the Amphibia, the lowest group 

 with true internal nares, their development supports the view that they are a new 

 formation. 



For a general account, 'with opposite conclusions, see Balfour, 'Comparative 

 Embryology,' ii. pp. 531-538. 



t The exoskeletal peculiarities of the Teleostomi may be summarised in the phrase 

 " dermal ossification." On the bodj', where flexibility is a requirement, juxtaposed 

 rhombic bony plates or " ganoid scales," arranged in parallel longitudinal and oblique 

 series, are developed ; these are found in the more generalised forms, and in the more 

 specialised ones may be variously modified or may disappear. Other parts of the fish 

 become strengthened and protected by membrane- bones, and whether we examine the 

 more primitive members of either the Chondrosteo-Teleostean or the Crossopterygio- 

 Dipneust series, we find in each the same cranial roof-bones, paired parietals, frontals, 

 nasals, post-frontals, and supratemporals, also the parasphenoid covering the basis 

 cranii, the prajmaxillaries and maxillaries (probably originally overlying labial car- 

 tilages like those of the Selachians), the dentary, angulare and splenial, sheathing the 

 Meckelian cartilage, the circiTmorbitals, postorbitals and prseoperculum, the opercular 

 and subopercular bones, protecting the branchial chamber, and finallj^ a series of bones 

 overlying the pectoral arch and connecting it with the cranium, post-temporal, 

 supra-cleithruiTi, cleithrum, and clavicle. 



X The Teleostomi have typically either a lung or its homologue, the air-bladder. 



§ The dermal rays of the Teleostomi differ froin those of the Selachii in being 

 more or less ossified ; in order to retain their flexibilitj^ they have become segmented. 

 In the two subclasses the dermal rays occupy exactly the same position with regard 

 to the muscles and the endoskeletal supports, and they appear to me to be unques- 

 tionably homologous throughout both a-roups. For another view, see Goodrich, 

 Quart. Journ. Micr. Sci. xlvii. (1904) p. 464. 



II In the Teleostomi, in addition to the neural arches which are present in the 

 Selachii, we find a series of paired elements which are the dorsal equivalents of the 

 ribs. In the living Chondrostei and Dipneusti these supra-neurals are attached 

 proximally to the neural plates (basi-dorsals) on each side of the longitudinal 

 ligament, and thej^ meet above to form the neural spines. The ribs in the 

 Teleostomes appear not to be homologous with those of the Selachians ; in the 

 former group thej' lie internal to the muscles and bound the abdominal cavity, 

 whilst in the latter thej^ lie between the dorsal and ventral muscles, corresponding 

 rather to the intermuscular bones of manjr true Fishes. That the Teleostome ribs 



