THE SHOULDER-GIRDLE IN FISHES. 41 



supra-clavicle in one piece. The fifth plate (the first is the post-temporal), reaches higher up, is, 

 like the last, overlapped by the dorsal-line plates, and grows downwards some distance within the 

 large infero-lateral plate connected with the ventral fin. There are three more such plates in 

 this species, (G. leiurus], the last being somewhat smaller; they overlap each other from before 

 backwards. The fifth and succeeding ganoid plates answer to the post-temporal, supra-clavicle, 

 and clavicle all in one piece; they are united by strong bony pegs, and are perforated by 

 mucous ducts ; and they answer, in the mucous region, to the lateral-line bones of the ordinary 

 Teleostei. This division of the lateral plates is very different from what is seen in Calliclithys, 

 in which the plates divide close below the mucous ducts (see Plate I, fig. 9, s. 1., 1 4). 



I see no plates of bone in this species that belong to the middle part of the second cincture, 

 the bones of which are evidently absent because of the overgrowth of the clavicle and inter- 

 clavicle. Nearly the whole of the thorax and abdomen is invested below by two pairs of huge 

 infero-lateral ganoid plates. These are the inter-clavicles (fig. 5, A, B, i. cl.) and the plates that 

 are correlated to the ventral fin; thus, the Shoulder-girdle and the Hip-girdle attract to 

 themselves, below, large ganoid, dermal Breast-plates, that are serially homologous, and that 

 equally run wild over the lower part of several Somatomes. My business, at present, is with the 

 anterior pair (i. cl.) ; and these will be found to have grafted themselves upon the true Shoulder- 

 girdle, showing that they are as much correlated to this part of the endo-skeleton as the clavicles 

 themselves. If these bones be compared with those of the Sturgeon (Plate I, figs. 6 8, i. cl.) 

 it will be seen that they are almost precisely alike; the ganoid, outer portion sending inwards, 

 from its upper edge a thin but broad lamina ; but there is an important difference. In the 

 Sturgeon this inner part is subcutaneous, and is a continuation of the bony hinder wall of the 

 gill-opening, the upper part of which is formed by a similar plate growing from the clavicle. In 

 the Stickleback it is an aponeurotic plate, which lies within the muscles of the shoulder and 

 becomes soldered to the lower edge of the pra3-coracoid. 



The true Shoulder-girdle is of very great relative breadth in Gasterosteus ; and it is 

 anchylosed above, in front, and below to the large exo- skeletal plates ; but as these aponeurotic 

 laminae are connate with the ganoid plates, this little Fish agrees with the Siluroids, and not with 

 the Ostracioids, in which the ganoid plates are entirely differentiated . from the aponeurotic ossifi- 

 cations : this is a great stride towards the typical Teleostean skeleton. The broad, thin scapula 

 (fig. 5 A, sc.) is well ossified, save towards the coraco-scapular synchondrosis ; very much of its sub- 

 stance disappears in the middle, for its " fenestra" (sc. f.) is very large ; this is a transversely oval 

 space. The general outline of this plate is five-sided, narrowing towards the top, both fore and aft ; 

 its largely extended base is nearly straight, and is directed a little downwards as well as backwards. 

 A considerable space of unchanged hyaline cartilage intervenes between the base of the scapula 

 and the upper surface of the coracoid, and in this region the scapula has a core of soft cartilage. 

 The arcuate front margin is anchylosed to the posterior plate of the clavicle; the supero-posterior 

 margin is straight, and, like the posterior edges of- the brachials, is considerably thickened; the 

 infero-posterior margin is very thin, joins the upper part of the selvedge by a very obtuse angle. 

 and, being the glenoid region, it articulates with three of the brachials. Behind the supero- 

 posterior margin there is a band of flat-celled hyaline cartilage, which is continued down behind 

 the brachials, and it is only partly divided by a curving inwards of the cells towards each inter- 

 osseous space. The praa-coracoid (p. cr.) is a transversely oblong bar, the upper margin of which 

 is formed by the double ectosteal plate which clamps and encloses the lower part of the cartila- 

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