BRIEF REVIEW OF THE SYSTEM OF LIFE. 



417 



the Sharks (Fig- 375), but non-vertebrated or homocercal in many later kinds (Fig. 366), 

 except in the embryonic state. Teeth (Figs. 372, 373) labyrinthine in interior structure 

 (Fig. 374, a cross-section), a feature which is more strongly marked in the teeth of ancient 

 Amphibians (the Labyrinthodonts), which geologically succeeded to the Ganoids. 



The Ganoid tribe includes : — 



The Placoderms, an aberrant type, having the head and anterior part of the body 

 covered with a shield made up of plates, as represented in figures of Pteraspids, Cepha- 

 laspids, Asterolepids, etc., on pages 566, 624. The posterior part of the body has scales, 

 which admit of free movement for sculling locomotion. The pectoral fins are large arms 

 in the Asterolepids, fitted apparently for crawling over muddy surfaces left by the retreat- 



ing tide. 



3(56-874. 



Ganoids (excepting 369, 370). — Fig. 366, Tail of Thiissops (x |) ; 367, Scales of Chirolepis Traillii (x 12) ; 

 368, id. Palaeoniacus lepidurus ( x 6) ; 368 a, under-view of same ; 369, Scale of a Cycloid ; 370, id. of a 

 Ctenoid; 371, part of pavement-teeth of Gyi-odus umbilicus; 372, Tooth of Lepidosteus; 373, id. of a 

 Cricodus; 374, Section of tooth of Lepidosteus osseus. 



The Crossopterygians, or those having in the pectoral fin, like many Dipnoi, a thick- 

 ened finger-like axis, with reference to which the rest of the fin is like a fringe, and 

 thence the name of the group. (Sthenopterygians, referring to the strengthened axis of 

 the fin, would be better. ) Holoptychius, Onychodus, Glyptolepis, Rhizodus, Osteolepis, 

 are some of the ancient genera ; and Polypterus, of the Upper Nile, is a related genus. 



375. 



Palaeoniscus Freieslebeni (x J), Permian. 



The Palceoniscoids, in which the pectoral fins have no thickened axis, besides other 

 peculiarities, as in Palaeoniscus (Fig. 375), Chirolepis, Eurynotus, etc. 



The PycnorJonts, having the palate paved with blunt rounded molar-like teeth, as in 

 Pycnodus, Gyrodus (Fig. 371), etc. 



3. Dipnoans or Lung-fishes. — These fishes, of which the species of Lepidosiren and 

 Ceratodus are living representatives, have both gills and lungs, the air-bladder being 

 cellular, so as to have functional value as a lung — a characteristic that enables the 



DANA'S MANUAL — 27 



