THE SHOULDER-GIRDLE IN FISHES. 33 



Ordo " PLECTOGNATHI.' 



Example 1. Os f radon auritus, Shaw. 



My specimen of this Trunk-fish measures two and a half inches in length, and is therefore 

 about half grown. I have been long familiar with the complete differentiation of the ossifica- 

 tions of the " cutis vera" from those of the looser fibrous layers within in this genus. The 

 Woodcut (figs. 4 A, 4 B), shows the Shoulder-girdle and its extensive system of subcutaneous splints, 

 magnified three diameters. In fig. 4 A these latter bones are shown from the right side, the view 

 being from within. Tig. 4 B shows the Shoulder-girdle moiety, with the large splint-bone that 

 underprops it. Professor Owen ('Osteol. Catal. Hunt. Mus.,' vol. i, p. 81, No. 362) aptly 

 compares the bones of Orthagoriscus mola (the Sun-fish) to decayed wood. Those of the Trunk- 

 fishes have this same touchwood consistence, totally unlike those of the Balistidse, which have 

 them much more dense and ivory-like. The post-temporal and supra-clavicle (fig. 4 A, p. t., s. cl.) 

 together form a thin triangular plate, convex on the outside, where it adheres to the hexagonal 

 plates, and concave within. The post-temporal has a thick, incurved edge above ; and this part 

 thickens still more where the bony fibres spread over the delicate cap-like " epiotic." The supra- 

 clavicle (s. cl.) has a small spur in front, and ends in a strong peg below ; in front it is attached 

 to the outer occipital region ; its hinder margin is lunate, and below it is clamped by a strong, 

 rod-like splint, which passes across to the " basis cranii," just where the " parasphenoid" interdigi- 

 tates with the " basi- occipital." I shall provisionally call this bone the " prse-clavicle" (fig. 4 A, pr. 

 cl.) ; it curls up behind the supra-clavicle externally, and separates it from the broad shell-like top 

 of the clavicle. I have described a similar splint-bone in Glorias (p. 29), which runs across from 

 the junction of the clavicle with the supra-temporal to the under surface of the " atlantal" centrum. 

 These bones may be true homologues ; they are at least the same serially. Between this bar 

 and the broad top of the clavicle, in front, there is a rod of cartilage (p.), which seems to answer 

 to the so-called scapula of the Lepidosiren, and is probably an autogenous " pharyngo- 

 branchial." Between the supra-clavicle and the clavicle there is, behind, the pedate upper end 

 of the upper post-clavicle (u. p. cl.), so that there are here four bones set on to each other by a 

 peculiar kind of carpentry. The clavicle (cl.) is the largest bone of the seven that are built 

 above, below, and around the true Shoulder-girdle ; its anterior concave surface is of great breadth 

 greatest where it appears narrow in the side view,, as in the figure. This back wall to the 

 branchial cavity is transverse to the axis of the body. The high triangle, with its incurved 

 posterior margin, which runs down to the abdominal line, grows out of the middle of the anterior 

 plate, so that the whole bone is three-winged. Above, the clavicle forms an elegant thin shell 

 below the prae-clavicular bar ; this bone is widest in the middle, where it runs across the 

 thoracic cavity to underprop the obliquely oval doorway through which the pectoral fin passes. 

 The external lamina becomes very narrow, but reaches the base of the bone ; the inner wing 

 stops midway on the edge of the vertically longitudinal plate ; the base of the latter is thin, but 

 is composed of two laminae. The upper post-clavicle (u. p. cl.) is a considerable bar of bone, 

 dilated in front (above), where it wedges itself in between the prse-clavicle and clavicle, and 

 5 



