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of the cartilage passing wedge-like between the processes and its backward motion being arrested by 

 its wedging in between them. That part of the mesethmoid that lies on either side of the median keel, 

 forms part of the floor of the nasal pit, but the nasal sac rests but little upon it, being crowded off 

 it by the rostral. On the dorsal surface of the posterior portion of the mesethmoid, on either side, 

 the frontal rests, the articulating edges of the two bones being often more or less dovetailed together. 

 The frontal of either side extends forward to the base of the corresponding mesethmoid process, the 

 two frontals entirely covering the posterior part of the mesethmoid excepting only a small median 

 portion. 



The mesethmoid of Scorpaena, although undoubtedly a so-called primary bone, consists of 

 two distinctly different portions. One of these portions is a thin dense layer of superficial bone. The 

 other portion is a deeper one, of quite different appearance, which underlies the central portion only 

 of the superficial portion, and there replaces portions of the cartilage of the skull. I have made no 

 attempt to study the development of these two components of the bone, or even to determine, other 

 than most superficially, their character. The investigation has however involved the examination 

 of several series of sections of specimens from 45 mm to 55 mm in length, and in these sections the 

 condition of several of the cranial bones was more or less carefully noted. In these specimens the 

 deeper portion of the mesethmoid of the adult, the portion that replaces parts of the cartilage of the 

 skull, has not yet begun to develop. The superficial portion of the bone is represented by a thin plate 

 that lies closely upon the cartilage of the skull, without intervening membrane, and must be primarily 

 wholly of perichondrial origin; but this perichondrial plate receives, at certain places, accretions, or 

 additions, to its outer surface, and these accretions, although they present in sections exactly the sarae 

 appearance as the perichondrial plate, seem to be of purely membrane origin. This is particularly 

 noticeable, in my specimens, along the lines of the articulation of the mesethmoid with the frontals, 

 and in the mesethmoid processes. These latter processes rise from and are directly and unbrokenly 

 continuous with the layer of perichondrial bone, but they are so important, and so wholly out of all 

 relation to existing cartilage that they must be largely of purely membrane origin. Otherwise, there 

 must be here an important fold in the perichondrial tissues which occupies the place of a cartilaginous 

 process found in other earlier forms, and this tissue must give origin to perichondrial bone without 

 the related development of any cartilage. 



The superficial portion of the mesethmoid of the adult, of dermo-perichondrial origin, as above 

 explained, is in direct contact, on either side, with a corresponding portion of the ectethmoid. Anteri- 

 orly it is in contact with the dorsal limb of the vomer. The deeper part of the mesethmoid, of endosteal 

 origin, is every where in contact with, and replaces portions of the antorbital cartilage, and does not 

 extend ventrally through that cartilage. 



ECTETHMOID. 



The ectethmoid, on either side, is the prefrontal of many authors, the parethmoid of Swinner- 

 ton's descriptions of Gasterosteus, and the preorbital ossification of my own descriptions of Amia and 

 Scomber. It has, in Scorpaena, a dragon-wing-like appearance, and consists of a wing, an arm that bears 

 that wing, and a small bit of Shoulder or body that bears the arm and supports the mesial edge of the 

 wing. The wing is a thin plate of bone, concave posteriorly and convex anteriorly, which projects upward 

 and backward, in a curved line, from the dorsal surface of the arm, and is fused in the basal portion 

 of its mesial edge with a part of the body of the bone. The wing of the bone develops without being 



