12 Mr. A. S. Woodward on some 



Museum bj a fine specimen from the Bavarian Lithographic 

 Stone (no. 37023), shows that most of the scales in this genus 

 are united above and below by jagged sutures — a feature, so 

 far as the present writer is aware, unique among fishes. The 

 new Oxfordian specimens from Peterborough not only agree 

 in exhibiting this peculiarity, but also in the general aspect 

 of all the other parts which can be compared. Although the 

 caudal fin is wanting in all these fossils, there can therefore 

 be no doubt that they belong to the fan-shape-tailed Mesturus 

 rather than to the fork-tailed Gyrodus^ which is the only 

 other genus displaying much resemblance. 



With regard to specific characters, it may be remarked 

 that these are usually sought among Pycnodonts in the tritoral 

 dentition. The precise characters of the teeth of M. verru- 

 cosus^ however, are unknown, and the dentition in three of the 

 Oxfordian specimens now described is so remarkably similar, 

 notwithstanding the striking differences in the arrangement of 

 the bones of the cranial roof and the variation in the tuber- 

 cular ornament, that it is diflScult to form any judgment as to 

 nomenclature. The specimen of M. verrucosus^ however, in 

 the British Museum is characterized by an irregular enlarge- 

 ment of the tubercles along the anterior half of the course of 

 the " lateral line," while many of its scales display radiating 

 structural lines ; and both these features are wanting in the 

 new fossils from the Oxford Clay. We therefore appear to 

 be dealing with a new species, of which the dental characters 

 are described in detail on p. 8, and it may be appropriately 

 named Mesturus Leedsi^ in compliment to its discoverer. The 

 fossil no. 1 (Leeds Catalogue) must be regarded as the type 

 specimen, and the others are provisionally associated with 

 it until complete examples of each of these varieties are 

 discovered. 



IIL Conclusions. 



Two most important results are obtained from the new 

 specimens of Mesturus now described. The arrangement of 

 the cranial roof-bones of a Pycnodont fish is discovered for 

 the first time, and the true nature of the opercular apparatus 

 is determined. It is also interesting to observe a partial con- 

 firmation of the facts in regard to the arrangement of the 

 jaws, which were revealed two years ago by the study of the 

 Cretaceous genus Anomceodus *. 



* A. S. Woodward, " On the Genus Anomocndns, with Remarks on 

 the Structure of the Pycnodont Skull," Geol. Mag. [3] vol. x. (1893) 

 )). 187, pi. xvii. fig. 1. 



