PARENTAL CARE AMONG FREStl-WATER FISHES. ' 423 



CROSSOPTERYGIAXS. 



During the later Paleozoic period and the succeeding Mesozic the 

 predominant fishes were forms distinguished by the axial prolonga- 

 tion of the skeleton of the paired fins, pectoral as well as ventral. 

 The species and genera were many, and they represented a number of 

 families. They early disappeared — during the later Mesozoic — from 

 the waters of countries later to be inhabited by civilized men, but in 

 others the race or '' phylum " must have been continued, for to-day 

 relatives are still living in the fresh waters of the African c-^|^inent, 

 and there only. The last, though obviously akin to the ancient fishes, 

 are not near relatives, but in some respects quite different. Modern 

 naturalists concur in the ojnnion that many old forms and the new 

 constitute a great comprehensive group to which the name of Cros- 

 soptenjylans {Crosxapterygii) has been given. By k)me that group 

 is called a subclass; by others a superorder or an order. In the belief 

 that it is a natural group agreement is universal. 



All, extinct and recent, had or have the paired limbs developed 

 round an axial extension of the paired fin skeleton, so that the fins 

 are " lobate."' A distinct suspensorium of the loAver jaw is developed, 

 connected with the cranium hy a sim2)le suture. The skull is als-.) 

 characteristic in that the upper jawbones are not distinct, as in 

 lypical fishes, but continuous with the cranium as in Amphibians. 

 The heart is preceded bv an arterial nniscular bulb, whose cavity is 

 beset with several longitudinal rows of valves. 



The cardiac character is of course known only from the living 

 forms of the group, and thev^ have been recognized as a distinct type 

 (CJadlstia). These Cladistiaus^ which have been estimated by some 

 ichthyologists as an order, are distinguishable by the peculiar pectorrd 

 fins. The base of each of those fins consists of a Y-shaped j^art artic- 

 ulating with a convex condyle of the scajDular cartilage, and between 

 the forks of the Y-shaped element is a broad cartilaginous plate. On 

 the hind edge of this many ray-like bones or actinosts are set, and 

 these su])port the pectoral fin. 



It has been urged that this member manifests the nearest a[)pr()ach 

 from the fish side to the fore limb of a terrestrial vertebrate. The 

 ■' convex condyle of the scapula *' is su^^posed to be homologous with 

 the humerus, the Y-shaped element represents the radius and ulna, 

 the intervening cartilage the material for the cari)us, and the actinosts 

 the metacarpals. The group is consequently one of singular interest 

 to the morphologist. The interest is not lessened by the imperfection 

 oT the history of the group. While its ancient relatives died out 

 countless ages ago in the explored regions of the earth, in Africa 

 one branch of the group must have survived, and a nuuiber of species 

 still represent the order and th(> subclass in many an Afi-ican river. 

 These representatives all belong to one family, called the Polypterids, 



