450 NATURAL SCIENCE. Dec, 1893. 



scales by the mechanical conditions in the comparatively mobile 

 caudal region where they are found. 



It is well known that even in ordinary ganoid fishes with rhombic 

 scales, their peg-and-socket union is almost always wanting on the 

 tail, as if to ensure greater freedom of motion. So far as the present 

 writer is aware, however, no case has hitherto been noticed in which 

 firmly-united quadrangular scales are fixed at their upper and lower 

 borders by more than the peg and socket. It is, therefore, of much 

 interest to record that the scales of the Pycnodont genus Mesturus 

 (from the Bavarian Lithographic Stone and the English Oxford Clay) 

 are joined together above and below by a deeply dentated, interlocking 

 suture, exactly as observed in certain scales of crocodiles. Mesturus, 

 indeed, must have had an almost inflexible scale-armour. 



If further proof were required that rhombic scales in the Palaeozoic 

 fishes are more primitive than deeply overlapping cycloidal scales, 

 we need only refer to the case of the Palseoniscidae. In this group 

 the fishes with cycloidal scales always retain the thick rhombic ones 

 on the upper lobe of the tail, however much the aspect of the trunk- 

 squamation itself may have changed (i). It is therefore curious to 

 find that among the Devonian fringe-finned ganoids, those with the 

 most primitive fins (Holoptychiidae) have the scales round and deeply 

 overlapping, while many of those with more specialised fins 

 (Osteolepidae) have the squamation rhombic. At the same time, it 

 must be remembered that the rhombic-scaled Megalichthys of the Coal- 

 measures gradually passes into the round-scaled Rhizodopsis of the 

 same age, the inner rib of each scale then degenerating into a small 

 tubercle ; and some of the round scales of the Devonian fishes show 

 this inner tubercle, which is likely to have arisen in the same manner. 



REFERENCES. 



1. Fritsch, A. — -Fauna der Gaskohle und der Kalksteine der Permformation 



Bohmens, vol. iii.,pt.2, 1893. 



2. Lankester, E2. R. — The Fishes of the Old Red Sandstone of Britain. Part i., 



Mon. Pal. Soc, 1870. 



3. Rohon, J. Y. — Die Obersilurischen Fische von Oesel. Mhn. Acad. Imp. Set., 



St. Pctershoiirg, vol. xli., no. 5, 1893. 



4. Ryder, J. A. — On the Mechanical Genesis of the Scales of Fishes. Proc. 



Acad. Sci. Philad., 1892, pp. 219-224. 



5. 'Wood'ward, A. S. — Catalogue of the Fossil Fishes in the British Museum. 



Part ii., 1891. 



A. Smith Woodward. 



