302 PKOCEEDINGS OF THE GEOLOGICAL SOCIETY. [Feb. 21, 



in the generic definition (Poiss. Foss. ii. p. 6) and in the description 

 of PI. gibbosus (ibid. ii. p. 165) are not Pycnodont but Lepidoid 

 (=Lepidosteid). The former family was thus made to include two 

 distinct types of dentition, associated, as will appear, with equally 

 distinct osteological characters. ThioUiere in a remarkable paper 

 " On the Jurassic Fossil Fish of Bougey " (BuU. Soc. Geol. France, 1858, 

 XV. p. 372) alludes to the vagueness of the definition of the Pyc- 

 nodont family, whence, he thinks, should be excluded Platysomus, 

 Tetragonolepis, Phyllodus, &c., whose reference to that natural group 

 is based, he says, " on insufficient or deceptive analogies." Cocchi 

 has since (Nuova Famigha di Pesci Labroidei, 1865) given good 

 reasons for removing Phyllodus to the Labroidei. The place of Te- 

 tragonolepis is undoubtedly among the Lepidosteidce. 



Professor Huxley having kindly given me access to the specimens 

 under his care, specimens obtained for the most part by Mr. Moly- 

 neux of Burton-on-Trent, I have had very favourable opportunities 

 for studying the genus Platysomus. I am also indebted to Mr. 

 "Ward of Longton, and Mr. Garner of Stoke-on-Trent for the libe- 

 rality with which they lent for examination many nearly perfect 

 specimens from their cabinets, and to Mr. Davis for his assistance 

 when examining those in the National collection. 



The Pycnodont family is characterized by teeth adapted for 

 crushing, which consist of a circular or transversely oval crown, 

 flattened above, and sessile on the bone to which it is attached ; or 

 of an obtusely conical crown which is broader than its peduncle of 

 support. 



The teeth in the Carboniferous genera which form the subject of 

 this paper present three types ; 1st, the sharply conical tapering 

 teeth common to most Lepidosteids ; 2nd, the blunted cones with 

 constricted necks ; and 3rd, a series of dental tubercles of peculiar 

 ultimate structure. 



To the species presenting the former type of dentition, I propose 

 to restrict the generic name Platysomus. The genera presenting 

 the second type are, equally with the former, but in a different di- 

 rection, transitional to the Lepidosteidce ; while the genus Amphi- 

 centrum, mihi, in which the dental tubercles occur, is distinct from 

 the Lepidosteids on the one hand, and, on the other, from the Pyc- 

 nodonts properly so called. 



Platysomus, Agassiz, partim. 



Body flat, broad. Head triangular, higher than long ; snout 

 sharply angular. Premaxilla small ; maxilla in a single piece ; 

 mandible slender, spatulate ; all three bones armed with fine, conical, 

 sharp teeth. Branchiostegal rays few, enamelled. Interopercular 

 wanting. No ventral fins. Dorsal and anal fins opposite; their 

 bases extended, and nearly equal in length. Tail heterocercal, equi- 

 lobate. Scales oblong, vertically striated, with moderately strong 

 lepidopleura. The marginal scales anterior to the opposite fins, 

 more or less modified. Notochord persistent ; arches ossified. 



The genus thus defined includes PI. gibbosus, rhombus, striatus (?), 



