OSTEOLOGY, ETC., OF SYNGNATHUS PECKIANUS.' 649 



Elasmobranchs.i This is apparently a secondary formation due 

 to the coalescence of the extremities, since in earlier examples 

 it is not present. Resting on this longitudinal bar, opposite 

 the intervals between the cartilaginous rays, are oval cartila- 

 ginous nodules (B.C.), each one of whichs upports a horny ray 

 (H. R.). These do not present any transverse segmentation 

 To each of these nodules are attached two muscles, a posterior 

 smaller one (Dep. M.), which by its contraction depresses the 

 ray, and an anterior (Er. M.), attached to the other extremity 

 of the nodule, which acts as an erector. 



The cartilaginous supports for the fins appear long before 

 there is any trace of ossification of the spinal column. 



As was stated before, the tail fin is heterocercal at first, and 

 passes tVirough nearly the same formative changes as the dorsal. 

 The urostylic cartilages are large, and become formed previous 

 to the completion of the diiferentiation of the horny rays : they 

 are comparable, to a certain extent, to the interspinous or sup- 

 porting rays of the dorsal. 



V. The Gills and Alimentary Canal. 

 The gills of the Lophobranchs have usually been described 

 as " tufted," from the supposition that they consisted of tufts 

 of filaments. Dr. Ryder'"^ has corrected this mistake for 

 Hippocampus. The diagram of a gill leaflet of Syngnathus, 

 given on Plate XLII, fig. 7, will show their real structure. 

 Coming ofi" from either side of the rachis are a number of 

 leaflets, gradually increasing in size from below upwards until 

 near the extremity, when they suddenly decrease. Four such 

 rows (only two are represented in the diagram) are arranged 

 on each rachis, forming, as it were, a rectangular pyramid 

 aflixed by its apex. As Dr. Ryder says : — " There is therefore 

 nothing at all in these structures which is not represented 

 homologically in the fish's gill of the ordinary type, since the 

 two series of vascular branchial appendages to each arch in 

 Hippocampus are perfectly comparable with the bifurcated, 

 vascular, branchial appendages of such a form as Salmo." 



^ F. M. Balfour, ' Comparative Embryology/ vol. ii, London, ISSl. 



' Loc. cit. 



