SCIENTIFIC SURVEY. 55 



The scapular arch is suspended directly from the skull. The 

 exo-skeleton is generally deposited iti the form of ganoid plates, 

 but there are in representatives of some families, oval or cycloid 

 scales, and the body is still more rarely naked, and the bony 

 plates absent. The optic nerves, like those of the plaigiostoms 

 are only connected by a commissure, and do not cross or decussate. 

 The bulbus arteriosus is muscular and provided vs'ith two or more 

 rows of valves, which, in one order are replaced by two spiral and 

 lon"-itudiaal valvular folds. The intestine has frequently, — but 

 not always, — a spiral valve. The branchial apertures are simple 

 fissures, or spiracles on each side as in ordinary fishes ; the bran- 

 chiae are free. An air bladder is present and communicates by a 

 duct with the intestinal anal. The ventrals are abdominal. 



This sub-class has been, for the present, divided into four orders, 

 as follows : 



Order ITolostei, Muller. 



( Whole Bones. ) 



This order embraces fishes which are provided with plates which 

 are either rhomboid and tiled, or oval and imbricated. The hyoid 

 (tongue) apparatus has one or many branchiostegal rays. The 

 centre of the vertebra? are either ossified or represented by a 

 persistent notochord, (spinal chord.) The neuro-apophyses 

 (Ganglions) and haemapophoses) arterial bulb, are always ossified. 

 The dorsal and anal fins are sustained by true dermo neural spines 

 articulated with the inter neural spines. The scapular arch is 

 suspended by two processes to the paroccipital and the mastoid 

 bones, and sustains well developed pectoral fins which are pro- 

 vided with many rays. The abdominal ventral fins, are also sup- 

 ported by several rays. The bulb of the aorta has several longi- 

 tudinal rows of valves. 



Order Placoganoidei, Owen. 



This order embraces those fishes that were, rather than those of 

 the present day, and their characteristics are studied from the 

 organic, or fossil remains, discovered by the geologist in his 

 researches into the formations of early epochs of the earth on 

 which we live. 



