WELLBURX : OX THE GENUS MEGALICHTHYS, AGASSIZ. 63 
the convexity being backwards ; its direction is from above down- 
wards and slightly backwards. Above it articulates with the 
squamosals (Sq.), and from this point it passes down behind the 
posterior border of the bones X and to meet the articular 
extremity of the mandible below, and behind are two bones (Op. 
and S.Op.). The former of these bones (Op.) represents the Oper- 
culum and the latter (S.Op.) the Suboperculum. 
The two cheek plates X and X^ are probably equivalent to 
the cheek cuirass of Lepidosteus (or as Dr. Traquair remarks of 
the same bones in Bhizodopsis)* to the posterior set of sub- 
orbitals in Lejndofus and in the Palaeoniscidae. By Agassiz f the 
bones X and P. Op. were considered to be the equivalent of 
the so-called pro-opercalum of Polypterus, while the lower one X^ 
he compared to the little bone fixed above the posterior edge 
of the maxilla in Salmonidae, kc, and which Mr. Parker con- 
siders to be the homologue of the malar bone of other vertebrata. 
The opercular bones (Op. and S.Op.) were largely developed. 
The operculum is a large square-shaped bone with its posterior 
superior angle much rounded, it is broader abo^'e than below and 
behind than in front. It is bounded above by the posterior half 
of the squamosals in front, and behind these by the lateral pair 
of suprateuporals ; in front is the preoperculum ; behind the 
bones of the shoulder girdle and below by a plate (S.O.), which 
it overlaps, the suboperculum. This latter bone is much narrower 
and has the anterior superior and posterior inferior angles much 
rounded. The bone is bounded above by the operculum, in front 
by the lower part of the preoperculum, behind by the clavicle, 
and below by a bone (L..J.) which is to be regarded as the most 
posterior of the lateral jugulars. (Plates XIII., XIV., and XY., 
Kg. C.) 
Jaws. — The maxillae are of an elongated triangular shape, 
the alveolar border being the longest and the posterior one the 
shortest. The greatest depth of the posterior expansion varies in 
* Trans. Roy. Soc. Edinburgh, Vol. XXX., p. 177. 
t " Poisson's Fossiles," Vol. II., part 2, p. 92. 
