. .. 



64 PISCES. 



the caudal region of an undetermined species from the Upper 

 Muschelkalk (Lettenkohle) of Bavaria. The Austrian specimen 

 proves that the head of the original early Mesozoic Ceratodus 

 exhibits the same arrangement of bones as that of the living 

 fish from Queensland, only differing in its higher degree of 

 ossification ; for in C. sturi the cheek- plates which remain carti- 

 laginous in C. forsteri are all bony, and the visceral arches are 

 much more completely ossified. The tail from Wiirtemberg 

 proves also that the dermal rays in the caudal fin are stouter 

 than those of the existing fish. 



Compared with its ancestors, the modern Ceratodus (Epi- 

 ceratodus) is thus slightly degenerate in the ossification of the 

 skeleton. The same remark applies to the existing sturgeons 

 and batrachians. 



ORDER 2. ARTHRODIRA. 



The systematic position of this extinct order is indeed 

 doubtful ; but in the present state of knowledge the remark- 

 able fishes contained in it are most satisfactorily interpreted on 

 the supposition, that at the period of their dominance in 

 Palaeozoic times the Dipnoi attained extreme specialization and 

 developed a tribe of armoured forms. The Devonian Coccosteus 

 and its allies, which constitute the order, are fishes with both 

 head and trunk armoured ; and in the more specialized genera 

 the shield of the abdominal region articulates with the head- 

 shield in ginglymoid facettes which admit of free motion. The 

 name ARTHRODIRA (joint-neck) has reference to this remark- 

 able character. There is no trace of a hyomandibular bone in 

 the most exquisitely preserved specimens ; and all appearances 

 in certain well-preserved American skulls of .Dinichthys are in 

 favour of the supposition that they are truly autostylic. The 

 upper dentition is confined to the elements of the pterygo- 

 palatine arch and the vomers : and only one ossification has 

 been observed in each ramus of the mandible. The jaws of 

 Dinichthys are most nearly paralleled by those of the existing 

 Lepidosiren and Protopterus; and the loose triturating plates of 

 Mylostoma only differ in unessential particulars from those of 

 Ceratodus. The notochord is persistent, and in Coccosteus the 



