3H VERTEBRATA. 



Mesozoic epoch, the Ganoid fishes begin to lose that predomi- 

 nant position which they before occupied, though they continue 

 to be represented through the whole of the Mesozoic and Kain- 

 ozoic periods up to the present day. The Ganoids, therefore, 

 are an instance of a family which has endured through the 

 greater part of geological time, but which early attained its 

 maximum, and has been slowly dying out ever since. Towards 

 the close of the Mesozoic period (in the Cretaceous period) the 

 great order of the Teleostean or Bony fishes is for the first time 

 known certainly to have made its appearance. The orders 

 of the Marsipobranchii, Pharyngobranchii, and Dipnoi have 

 not left, so far as is known, any traces of their existence in 

 past time. Judging from analogy, however, it is highly pro- 

 bable that the two former of these must have had a vast anti- 

 quity, and it is not impossible that trie so-called "Conodonts" 

 from the Lower Silurian Rocks of Russia may yet be shown to 

 be the horny teeth of fishes allied to the Lampreys. At pre- 

 sent, however, the weight of evidence is in favour of looking 

 upon these problematical little bodies as probably referable to 

 some of the Invertebrata. 



These so-called " Conodonts " are microscopic in their 

 dimensions, and have the form of " minute, glistening, slender, 

 conical bodies, hollow at the base, pointed at the end, more or 

 less bent, with sharp opposite margins" (Owen). They show 

 no trace of dental structure, and Professor Owen concludes 

 that they " have most analogy with the spines, or hooklets, or 

 denticles of Naked Molluscs and Annelides." 



It is also to be borne in mind that, though it has not yet 

 been possible to definitely refer any fossil fishes to the order 

 of the Dipnoi, recent discoveries have rendered it extremely 

 probable that some well-known extinct types really belong to 

 this order. Thus, the great " Barramunda " ( Ceratodus Fosteri) 

 of the rivers of Queensland would seem to be truly referable to 

 the Triassic genus Ceratodus, in which case this latter must be 

 removed to the DipnoL This remarkable fish also presents 

 some striking points of resemblance with certain extinct 

 Ganoids, such as Dipterus. Upon the whole, therefore, there 

 are good grounds for accepting Dr. Giinther's suggestion that 

 the Dipnoi should be regarded as a mere sub-order of the 

 Ganoids. 



In the following chapter are given the orders of the Fishes, 

 with the leading characters and geological distribution of each. 

 The order, however, of the Pharyngobranchii (comprising only 

 the living Lancelet), and that of the Marsipobranchii (compris- 

 ing the Lampreys and Hag-fishes), may be here dismissed, as 



