380 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [JunC 23, 



stedt, Wagner, Heckel, Thiolliere, Pictet, Costa, Knerr, Sauvage *, and 

 Liitken have all written on the Pycnodonti, and have contributed 

 much information in developing the characteristics of the family, and 

 rectifying some of the generic characters assigned by Agassiz. Many 

 more species have also been added, exemplified by specimens of rare 

 perfection, as may be seen by consulting the beautiful plates given in 

 the vrorks of Thiolliere and Heckel. Perhaps the latter author has 

 contributed more than any other observer to the typical characters of 

 the several genera ; but as his work is limited to specimens found in 

 the Austrian dominions, other writers must be consulted for species 

 found in other parts of the Continent. The genus Oyrodus is dis- 

 tinguished from the other Pycnodonti by the form of the tritoral teeth, 

 the deeply forked tail, the solidity of the scales, some peculiarities 

 in the vertebral apophyses, and by the presence of scale-ribs both 

 before and behind the dorsal fin. The peculiar mechanism of these 

 scale-ribs was first interpreted by myself in a paper communicated 

 to the Geological Society in 1849 f. Thiolliere has fully compre- 

 hended and clearly described this scale-structure peculiar to the 

 Pycnodonti % ; but Dr. Liitken speaks of " the peculiar manner in 

 which the scales are interlocked and attached to those ribs " §, as if 

 they were independent organs, whereas the rib or fillet is com- 

 posed of the thickened margins of the scales, homogeneous in 

 structure, and inseparable from them except by fracture, and each 

 of these so-called dermal ribs is made up of as many pieces as there 

 are scales in the dorso-ventral series. Heckel is of opinion that 

 these dermal ribs are analogous to the so-called Y-shaped ossicles 

 present in the ventral region of some recent fishes of the Clupeoid 

 family, only more largely developed, and he names them ridge-ribs 

 and keel-ribs (Firstrippen und Kielrippen ||) — a solution originated by 

 Prof. Agassiz ^, but since abandoned in favour of the explanation 

 which I have suggested. Having made a careful examination of a 

 much larger number of specimens, and in more perfect condition 

 than were available in 1849, 1 find no reason to alter or modify the 

 observations I then published ; on the contrary, I find them con- 

 firmed by indisputable evidence. Heckel and Thiolliere are both in 

 error in describing the dental formula of the lower jaws of the Pycno- 

 donti as limited to four rows of tritoral teeth on each side. I have 

 several mandibles from the Oolite of Stonesfield and the Jura beds of 

 Soleure, in which a fifth row of small teeth occurs on the inner 

 margin of the jaw ; and Thiolliere has himself represented this row in 

 a specimen of Pycnodus Bernardi (Microdon of Heckel) figured in 

 plate v. fig. 2 of his work on the fossil fishes of Bugey. 



* ' Catalogue des Poissons des Formations Secondaires du Boulonnais,' par 

 Emile Sauvage, 1867. M. Sauvage having used the term Eulepidotce in his 

 pubHcation, it becomes incumbent on me to change the somewhat similar name 

 Eulejpidotus given to a section of the Lepidoti in a paper read on the 17th of 

 June 1868 : I propose to substitute for it the name Heterolepidotus. 



t Quart. Journ. Geol. Soc. 1849, p. 330. 



+ 'Poissons Fossilesdu Bugey,' p. 12. § 'GeologicalMagazine,' vol. v. p.431. 



II ' Beitrage zur Kenntniss der fossile Fische Oesterreichs,' p. 10. 



^ ' Poissons Fossiles,' vol. ii. pt. 2. p. 182. 



