14 SHOULDER-GIRDLE AND BREAST-BONE. 



coracoid creeps, snail-like, to the very edge of the clavicle (see fig. 7, p. cr., cl.), but stops short 

 there : this has its morphological meaning, and it is noteworthy how true these ganoid plates of 

 the Sturgeon are to their endo-skeletal correlates, notwithstanding their slight specialisation from 

 the ordinaiy bony plates of the body generally. 



The bony grains seen behind the great dermal shoulder-plates in the Sturgeon are, with the 

 exception of the " supra-clavicle," in a very generalised condition (see Huxley and Hawkins's 

 ' Atlas,' plate 5, fig. 3<z) ; there are none that can be specially described as " post-clavicle : " in 

 the Ganoid Fish with a bony endo-skeleton these can be determined. It will perhaps be said that 

 what I have described as single plates in the Sturgeon are in reality separable into an outer and 

 an inner layer (a " ganoid " and a true skeletal part) in the embryo : to this I answer yes, 

 granted ; but this arises from the very manner of the development of these plates, as I shall show 

 in the case of the dermal bones of the Chelonia (Plate XII); for where a thick stratum of 

 fibrous tissue is undergoing ossification (" parostosis "), the calcified tissue is always in laminae, 

 which are more or less distinct at first ; these may coalesce entirelv, and the bony matter grow 

 peripherad, so as to leave a very thin " quick " for the equally thin cuticle, and it may at the same 

 time grow centrad, working its way into the subcutaneous tissue, and . shooting along aponeurotic 

 septa. In the higher types of Fishes the bones that are correlated to the endo-skeleton are, for 

 the most part, largely extended, separate ossifications of this inner web ; whilst the sub-peripheral 

 stratum is ossified from many hundreds of points ; which points form the nuclei of the exquisite 

 " cycloid " and " ctenoid " scales, that have no more special relation to the endo-skeleton than 

 what is to be seen in the " placoid " grains and thorns in the skin of the Sharks and Rays. 



B. Ganoids with well-ossified endo-skeleton. 

 Example 1. Potypterus Bichir, Geoff. 



In describing other instances of the Shoulder-girdle in the true Ganoids, I shall make 

 free use of Professor Huxley's masterly Memoir on this Order, 1 referring to his figures, and 

 endeavouring to give a correct nomenclature Avhere the author wise in his doubts has left the 

 terms in a provisional condition. 



At page 22, two figures (16 and 17) are given (after Miiller, but " somewhat differently 

 named"). In the upper view the "post-temporal" plate is lettered s. s., it answers to the 

 so-called " supra-scapula " of the Teleostei : it is a mere supero-lateral ganoid bone ; the first of 

 the kind belonging to the trunk. Articulating with this bone, by a hinge-joint, is another broad 

 but strong bone, which has scarcely any ganoid structure, for it lies deeper ; it is seen in shade in 



1 ' Memoirs of the Geological Survey of the United Kingdom/ " Figures and Descriptions illus- 

 trative of British Organic Remains," Decade 10 ; London, 1861. I would also refer the reader to J. 

 Miiller's work on ' Fishes/ ' Ueber den Ganoiden und den natiiraliche System der Fischc/ 1846. 

 Also to the works of Agassiz, viz., his " Vieux Gres Rouge," and his " Recherches sur les Poissons 

 Fossiles,"1844; to Hugh Miller's 'Old Red Sandstone/ 1841; and to Professor Pander's work, 'Ueber 

 die Saurodipterinen, Dendrodenten, Glyptolepiden, und Cheirolepiden des Devonischen Systems/ 1860. 

 Moreover, as my space does not admit of figures to all my instances, I must refer the reader to the 

 beautiful skeletons of Polypterus, Amia, and Lepidosteus in the Hunterian Museum. 



