ON THE SKELETON OF LEPIDOSTEtJSi 44S 



24. The Skeleton of Lepidosteiis, with remarks on the origin 

 • and evolution o£ the lower Neopteryoian Fishes. By 

 C. Tate Regan, M.A., F.R.S., F.Z.S., Keeper of 

 Zoology in the British Museum (Natural History). 



[Received April 24, 1923 : Read May 29, 1923.] 



(Text-figures 1-8.) 



Although a detailed and well-illustrated account of the skeleton 

 of Lepidosteus was given by Agassiz in 1833 (Poissons Fossiles, 

 vol. ii.), the nomenclature makes his description very difficult for 

 a present-day student to follow, and it has been ignored by most 

 modern authors. In his memoir on the development of the skull 

 of Lepidosteus osseus (Phil. Trans. 1882), Parker has given a 

 description of the skull of the adult fish which is in some i-espects 

 less complete and less accurate than that of Agassiz. More 

 recently Collinge (Proc. Birmingham Phil. Soc. viii. 1893) and 

 Allis (Internat. Monatsb. Auat. PhysioL xxi. 1905) have studied 

 Lepidosteus especially in relation to the sensory canal system. 

 Amia is much better known than Lepidosteus, but a proper 

 comparison of these two genera has never been made. 



I propose, therefore, to describe the skeleton of Lepidosteus, to 

 compare it with that of Amia, to discuss the systematic position 

 of these genera, and to put forward my views as to the origin and 

 evolution of the group to which they belong. 



I have examined skeletons of Lepidosteus platystomus, L. osseus, 

 and L. tristoechus ; all three species are essentially similar in 

 structure, differing from each otlier mainly in the shape of the 

 bones and the number of segments of the maxillary, due to 

 the fact that in L. osseus the snout is longer and narrower and 

 in L. tristoechus shorter and broader than in L, platystomus. 

 The nature of these difierences will be seen on comparing the 

 accompanying figures (text-fig. 1). 



Skeleton of Lepidosteus platystomus. 



Cranium (text-fig. 2). — The skull is elongate, narrowed forward, 

 depressed, with the upper surface flattish anteriorly and convex 

 posteriorly. It is well ossified, but a considerable part of the otic 

 ref^ion remains cartilaginous, and there is a large median ethmoidal 

 cartilage running the whole length of the rostrum. 



On the upper surface a transverse series of 6 dermo-occipital 

 plates is rigidly united to the parietals, pterotics, and epiotics, and 

 the small, simple post-temporal is very firmly attached to the 

 dermo-occipitalSj epiotic, and pterotic. Parietals, pterotics, and 



