i893. EXTINCT SHARKS AND GANOID FISHES. 



437 



cartilages of their limbs excessively abbreviated. It is, however, 

 stranger still to have to regard the Pala^oniscidas as primitive repre- 

 sentatives of the modern sturgeons ; and yet Dr. Fritsch, following 

 all who have deeply studied the subject, is fully convinced of the 

 necessity of this course. He describes the genus Trissolcpis, giving 

 the restoration copied below ; and this he regards as the type of a 

 distinct family to be placed in the same sub-order as Palceonisciis 

 itself. 



The Palaeoniscida;, as a general rule, conform to the type shown 

 in Fig. 2 ; being essentially sturgeons (as Dr. Traquair first pointed 

 out) covered with regular rhombic scales and provided with a complete 





i '" 



Fig. 2. — Restoration of Palo'Oiiiscus macropomus ; Permian, Germany. 

 {After Traquair.) 



gill-covering apparatus. In some cases, however, the scales disap- 

 pear except on the upper lobe of the tail ; and in a single instance, 

 briefly noticed by Dr. Traquair, the scales become deeply over- 

 lapping and almost or quite cycloidal. 



Now, the great interest of Dr. Fritsch's new contribution to the 

 subject consists in the fact that it is the first detailed and illustrated 

 account of one of these ancestral sturgeons possessing cycloidal 

 scales. Moreover, it is to be noted that in Tvissolepis the cycloidal 

 scales are confined to the trunk, while the rhombic scales persist on 

 the upper lobe of the tail. One and the same fish thus displays 



Fig. 3. — Restoration of Trissolcpis kounoviensis ; Permian, Bohemia. {After Fritsch.) 



features which were not long ago regarded as the essential characters of 

 distinct groups ; and the shape of the scales in future must only be 

 cited among the minor diagnostic points of the old " ganoid " fishes. 

 We are acquainted with Jurassic fishes, indeed, in which the anterior 

 scales are rhombic, ribbed, and united by peg-and-socket joints, 



