9 o 



ELOPIFORM FISHES 



sop 



Fig. 44. Pachythrissops laevis Woodward. Cranium in left lateral view. Composite 



of several B.M.N.H. specimens. 



The systematic position of Pachythrissops presents many problems, which stem 

 from the extreme primitiveness of this form (a phenomenon not unknown in this 

 work). An association of Pachythrissops with Allothrissops and Thrissops seems 

 unlikely and the suggestion that Pachythrissops is a stem ichthyodectid (Saint- 

 Seine 1949) is untenable. 



Some of the characters which unite Allothrissops and Thrissops with one another 

 and with the ichthyodectids include the following : 



i. Body form elongate with a small, deep head and an elongate anal fin 



originating anterior to a short-based dorsal fin. 

 ii. Pectoral fin with the outer rays somewhat enlarged and flattened. 

 iii. Vertebrae marked with a prominent longitudinal strut. 

 iv. Apparent lack of epipleural intermuscular bones, 

 v. Dentition in the form of a single row of small (Allothrissops) or prominent 



conical teeth (Thrissops). 

 vi. The preopercular sensory canal with many long branches extending postero- 

 ventrally in the ventral half of the bone. Although the individual branches 

 may not be seen in the Cretaceous ichthyodectids there is a series of many 

 pores near the ventral margin of the preoperculum indicating that there are 

 long branches from the main preopercular sensory canal, 

 vii. A stout basipterygoid process ; this is a retained character. 



