56 BOARD OF AGRICULTURE. 



Order Chondrostei, Muller. 

 Ossified Cartilage. 



The fishes embraced in tliis order, among which are the stur- 

 geons, have a skeleton, or frame work made up of both cartilage 

 and bones. The body is sometimes naked, but in most of the 

 species is covered with more or less interrupted rows of long or 

 ganoid plates of irregular form. 



There also many smaller plates and tubercles scattered on differ- 

 ent parts of the body. There are no true branchiostegal rays. 

 The vertebrse and their elements are cartilaginous. The skull is 

 also cartilaginous, but it is sometimes imperfectly ossified in front. 

 The scapular arch is suspended by two processes of the paroccipital 

 and mastoid bones. It supports two well developed ventral fins. 

 The ventral fins are also furnished with several rays. The bulb of 

 the aorta is furnished with several longitudinal rows of valves. 



Order Dipnoi, Muller. 

 [Double Spiracles, or breathing holes.) 



The bodies of the fishes embraced in this order are elongated 

 and covered with regularly imbricated cycloid scales. The centre 

 of the true vertebrae are cartilaginous, the notochord being persis- 

 tent. The continuous vertical fin, or fold encircling the posterior 

 part of the body, is sustained by articulated rays, immediately 

 connected with the spinous processes of the neurapophyses an*d 

 haemapophyses, which spines are osseous. The scapular arch is 

 suspended only to the exoccipital bone and supports on each side a 

 simple unjointed, or articulated ray on each side. The bulb of the 

 aorta is furnished internally with two spiral ridges, or valves. 



This brings us through the second class from the highest and to 

 the commencement of the third stage in the descending series of 

 the structural arrangement under consideration. The next is 



Sub-class Elasmobranchii, Bona. 

 {Plated gills or branchice.) 

 This class embraces the sharks and the bonj'- skates. They have 

 the endo-skeleton or vertebral column, and skull cartilaginous, or, 

 very imperfectly ossified. The exo-skeleton is developed in the 

 form of placoid granules. The brain is much more complex and 

 highly developed, than in the true fishes; the optic nerves are 



