9 8 4 



CLASS PISCES. 



oidal shape, presenting a striking resemblance in this respect to 

 the Platysomidce ; the caudal fin is of a completely masked hetero- 

 cercal type, and there are no fulcra to the fins. The notochord is 

 persistent, but the neural arches and ribs are ossified, and in the 

 later forms the heads of the latter (fig. 923) are enlarged, so as to 

 simulate portions of vertebral centra. The premaxillge are toothless ; 

 the conjoint palatines and vomer form a triangular bone (fig. 923), 

 carrying five longitudinal rows of oval or round molariform teeth, 

 while in the mandible the dentary bones, which form the extremity 

 of the symphysis, have two or four chisel-like teeth, and the splenials 

 are enlarged and carry on either side from three to five or more 

 rows of molariform teeth (fig. 923) opposed to those of the former. 

 Commencing with the typical genus Pycnodus, we find this, as now 

 defined, restricted in Europe to the Lower Eocene of Sheppey and 

 the Middle Eocene of Monte Bolca in Italy. There are only three 

 rows of teeth on either side of the mandible, of which the innermost 



is the largest ; while on 

 the vomer the teeth of 

 the three inner rows are 

 round, and those of 

 the two outer rows are 

 larger and oval. Other 

 characters are found in 

 the position of the 

 orbit, the large size of 

 the mouth, and the 

 superiority in the length 

 of the dorsal as com- 

 pared with the anal fin. 

 A fish from the Creta- 

 ceous of Brazil has been 

 referred by Professor 

 Cope to this genus. 

 The allied Palceobalis- 

 tum (fig. 923, d), in 

 which all the upper 

 teeth are subequal, 

 occurs in the Chalk 

 of the Lebanon, the 

 Cretaceous Pisolite of 

 Mont Aime in France, 

 and also in the Middle Eocene of Monte Bolca. The greater 

 number of the Cretaceous forms are, however, referable to the 

 genus Acrotemnus Ccelodus?- which comprises some species attaining 



1 The name Coelodus being preoccupied by Ccelodon, the term Acrotemnus, pro- 

 posed by Agassiz for detached teeth, is adopted. 



924. — Vomer of Acrotemnus gyrodoides ', from the 

 English Cretaceous. (After Egerton.) 



