282 Evolution and Distribution of Fishes 



Another palaeozoic elasmobranch group is that of the 

 Psammodontidae, the genera of which seem to have passed 

 wholly into a marine habitat, and are met with throughout 

 the Carboniferous Limestone rocks, but they became rare or 

 died out while the Coal Measures were being deposited. 

 In shape and dentition they seem somewhat to have re- 

 sembled the Pycnodonts of much later date and of different 

 ancestry. But there seem here to be two interesting cases 

 of parallelism in evolution. For starting from Eurynotus 

 (Fig. 1 6, p. 139) and even more primitive types that are 

 of chondrostean descent, a gradual condensation of the 

 body; modification of the dorsal and ventral fins; reduction 

 in size of the paired fins, and of the mouth; rounding, 

 shortening, and massing of the teeth till their almost com- 

 plete absorption in Cheirodiis, are continuous stages In a 

 process of condensing specialization observed from Eury- 

 notus through Mesolepis (Fig. 46) and Platysomiis (Fig. 

 45) to Cheirodus. Similarly starting from Lepidotus and 





Fig 46. Mesolepis scalaris, a chondrostean fish that is inter- 

 meditate between Eurynotus and Platysomus. From the Coal 

 Measures of central England. One-half natural size. (Reproduced 

 from Traquair). 



