21 



The subclasses of Ganoids as now characterized may be provisionally 

 divided into four orders, for which may be accepted the names of Holostei, 

 Placoganoidei, Chondrostei and Dipnoi. 



Order HOLOSTEI Mailer. 



The order of Holosteans embraces those fishes provided with plates 

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

 apparatus lias one or many branchiostegal rays. The centre of the 

 vertebras are either ossified or represented by a persistent notochord : the 

 neurapophyses and haemapophyses are always ossified. The dorsal and 

 anal fins are sustained by true dermo-neuial 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 provided with many rays. The abdominal ventral fins are also sup- 

 ported by several rays. The bulb of the aorta has several longitudinal rows 

 of valves. 



The order of Holosteans, as thus limited, embraces among recent fishes 

 only the families of Lepidosteoids, Polypteroids and Amioids. but the 

 number of its extinct representatives is very large. The order itself may 

 be subdivided into two suborders, to which may be given the names of 

 Rhomboganoids and Cycloganoids, there appearing to be a certain corre- 

 lation between the form and structure of the plates or scales and the rest 

 of the organization. The first two of the three orders above enumerated 

 are Rhomboganoids ; the family of Amioids belongs to the suborder of 

 Cycloganoids. 



Order PLACOGANOIDEI (Owen.) 



This group, which is now elevated to the rank of an order of the subclass 

 of Ganoids, was first accepted by Professor Owen, as a suborder of the 

 order of Ganoidei ; the name conferred on it by Owen has been retained as 

 the ordinal designation. Its representatives are confined to the earlier 

 age3 of the world's history. Their internal skeleton was either entirely 

 cartilaginous or the notochord was persistent. The head and anterior 

 part of the body were encased in a dense and compact helmet and coat of 

 mail. The posterior part of the body was covered with comparatively 

 small plates of various forms. 



posed that they should be considered as forming a single class, of which the 

 Reptiles and Amphibians of most modern zoologists may belong to one subclass 

 and the Fishes to another. Other naturalists consider the Amphibians as more 

 nearly related to the Fishes, and together separated from the true Reptiles by 

 the absence of the allantois which more or less envelops the embryo of the 

 latter. It might be equally appropriate to unite with Professor Owen all the 

 oold blooded Vertebrata in one class, for which the Owenian name of Hcewatocrya 

 may be retained, or to separate them into five or perhaps six classes, those of 

 the Reptiles, the Amphibians, the Fishes, the Elasmobranchiates, the Dermop- 

 teri, and possibly the Ganoids. 



[Jac. 



