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



contrasting, therefore, with the crushing-plates of Amphicentrum. 

 In Pycnodus, Thiolliere denies the existence of a maxillary, and finds 

 the analogue of the upper jaw in the palato -vomerine rows of crushing 

 teeth. "Wagner had figured, but not identified, a triangular plate 

 whose forward attachment is in the premaxillary region. This, 

 Thiolliere regards as a suborbital only, it would seem, on the ground 

 that its overlapping the prominent lateral mandibular teeth presents 

 difiiculties ; in his view, therefore, the palato-vomerines were naked. 

 Heckel* describes a specimen in which he finds a fragment of a 

 maxilla. It is to be supposed, therefore, that he regards this as 

 distinct from the bones bearing the superior mass of tritors. Am- 

 /pMcentrum gives the explanation of the arrangement: the outer 

 crest of the mandible is the higher of the two, and is external to 

 the inner maxillary crest, that, namely, which supports the denti- 

 cles. Doubtless the palatal plates were attached to the maxilla, 

 the latter having no free motion, as in most fish. Here, therefore, 

 the same difficulty is present as in Pycnodus ; but the overlap of the 

 mandible by the labial flap of the maxilla is unquestionable. The 

 triangular plate in Pycnodus becomes, then, the simplest form of 

 maxilla, an edentulous mobile plate covering in the oral cavity late- 

 rally. In Amphicentrum, therefore, the overlap is greater in degree 

 than, but similar in kind to, that partially developed in AecJimodus, 

 Dapedius, and Tetragonolepis. From the Pycnodonts Amphicentrum 

 is separated by the absence of teeth in the premaxillary and preman- 

 dibular regions, which rather resemble beaks than the same parts in 

 ordinary fishes f- The smaller gape and coincident change in place 

 and direction of the opercular apparatus further separate them, as 

 wiU be seen from ThioUiere's restoration (fig. 4), which I believe 

 to be correct, so far as the less perfect specimens in this country 

 enable me to judge. If to this be added the absence of a ventral fin 

 in Amphicentrum, and the less development of its vertebral arches, 

 the data seem sufficient to justify the reference of this genus to a 

 family distinct from, yet allied to, the Pycnodonts, and linking it 

 with the Lepidosteids. 



Comparison with ^chmodus, as the type of that group whose 

 association with the Pycnodonts Thiolliere justly condemns, shows a 

 divergence even more considerable than from Pycnodus, as will 

 appear from the restoration (fig. 3), and from the characters of the 

 teeth (Quart. Journ. Geol. Soc. vol. ix. p. 274, vol. x. p. 367). For 

 not only are the number and arrangement of the head-bones differ- 

 ent, but the structure and mode of articulation of the scales separate 

 this and allied genera from Pycnodus further than the latter is re- 

 moved in these points from Amphicentrum. 



The Pycnodont affinities of PZ. macrurus=Eurysomus, mihi, being 



* Sitzungsberichte der k.-k.' Akad. der Wissenschaften, Math.-nat. Classe, 

 vol. xii. 1854, p. 433. 



t The great height of the premaxillary ascending processes, relied on as evi- 

 dence of the mobility of the muzzle, is here associated with rigidity of the whole 

 oral structures, and even in Plat^somus the constant connexion of premaxillaries 

 and maxillaries in dislocated examples suggests that in that genus also the pro- 

 trusion of these parts was very limited. 



