316 PEOCEEDINGS OF THE GEOLOGICAL SOCIETY. [Feb. 21 , 



covered with rhombic scales, articulated by strong ribs traversing 

 their anterior margin internally. Dorsal fin equal to half the length 

 of the trunk. Anal fin also with an elongate base. Ventrals, when 

 present, small. Paired fins non-lobate. Eranchiostegal rays not 

 taking the form of broad plates. Notochord persistent. Arches 

 well ossified. 



I. Ventral fin wanting. 

 Plattsomid^. 



Teeth uniserial, conical, sharp. Palate-bones edentulous. — Platy- 

 somus, Agassiz, partim. 



Amphicenteid^ . 



* Dorsal and ventral margins sharply acuminated. Teeth in the 

 form of tuberculated plates on the maxillary, mandibular, and palato- 

 vomerine bones. Premaxillary edentulous. — AmpJiicentrum, n. g. 



EUETSOMID^. 



Teeth in the form of blunted cones, on a peduncle with a constric- 

 ted neck. — Eurysomus (^=Platysomus, Agassiz, ^arim.) . 



II. Ventral fin present. 

 Mesolepid^. 



Teeth similar to those of Eurysomus. — Mesolepis, n. g. ; Eurynotus, 

 Agassiz. 



Pyckodontid^. 



Teeth oval, hemispherical, or, if elongate, blunted cones. — Pycno- 

 dus, Mesodon, Gyrodus, &c., (except the Labroid forms of Cocchi). 



The arrangement here sketched does not affect the question which 

 further research may raise as to the true position of the genera 

 included under it. The group is transitional between the Ganoids 

 and Teleosteans, and is equivalent to the suborders, 1. Amiadae, 

 2. Lepidosteidee, 3. Crossopterygidse, 4. Chondrosteidse, and 5. 

 Acanthodidse. By Platysomus it approaches Palceoniscus and its 

 allies, while Pycnodus and Amphicentrum conduct to the Lahroidei 

 and Sparoidei. Prom the Pycnodonts the above definition excludes 

 Tetragonolepis and its allies by the negative character of absence of 

 lepidopleura, and by the important positive differences of cranial 

 structure already illustrated. 



The restoration of ventral fins by Agassiz in the genus Platysomus 

 was a matter of inference, which perfect specimens have disproved. 

 His generalization as to the non-appearance of apodal fish prior to 



in which parts of an osseovis skeleton are more distinctly preserved. To the 

 former belong the Lepidoids, to the latter Thrissops, Macropoma, Leptolepis, — a 

 miscellaneous assemblage. This classification is artificial and inaccurate, its 

 basis — the amount of surviving structures — being only in appearance a zoological 

 one. The character selected to define and designate the subfamily occurs in 

 more genera than those included by Quenstedt under it. 



