stedt, Wagner, Heckel, Thiolliere, Pictet, Costa, Knerr, Sauvage *, and Lutken 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 works 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 Gyrodus is distinguished 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†. Thiolliere has fully comprehended and clearly described this scale-structure peculiar to the Pycnodonti‡ ; but Dr. Lutken 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 composed 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, I find no reason to alter or modify the observations I then published ; on the contrary, I find them confirmed by indisputable evidence. Heckel and Thiolliere are both in error in describing the dental formula of the lower jaws of the Pycnodonti 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 Eulepidotoe in his publication, it becomes incumbent on me to change the somewhat similar name Eulepidotus 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.
† Quart. Journ. Geol. Soc. 1849, p. 330.
‡ 'Poissons Fossiles du Bugey,' p. 12.
§ 'Geological Magazine,' vol. v. p. 431.
|| ' Beitrage zur Kenntniss der fossile Fische Oesterreichs,' p. 10.
¶ ' Poissons Fossiles,' vol. ii. pt. 2. p. 182.
