192 BULLETIN OF THE UNITED STATES FISH COMMISSION. 



The most striking feature of all, however, in the organization of Hi]^- 

 pocampus is the downward flexure or bend of the head, which, together 

 with the shai)e of the latter, develops a most marked resemblance of 

 the forepart of the body to the head and neck of the horse, whence the 

 common name of the animal. Accomi^anying this leature the bones of 

 the snout have been prolonged so that the jaws are carried very far 

 forward, while the latter have themselves not undergone so much mod- 

 ification. The gills and opercular apparatus have also been much mod- 

 ified, the former in the adult consist of four pairs of arches with two 

 rows of i^innate, pyramidal, vascular branchial appendages resting on 

 and attached by their apices to their outer borders; these answer to 

 the branchial leaflets of other forms; the opercles articulate with the 

 hyomandibular behind by means of a distinct articular facet, and are 

 swung inwards and outwards on this articulation. At the upper border 

 of the opercle on either side and behind the auditory structures there 

 are placed the gill-openings, which are almost spiracular, and open up- 

 wards, the opercles being attached by their borders all round by mem- 

 brane, excex)t for a short distance at their upper posterior edges, where 

 the opercular efferent openings are placed. In that the water is forced 

 through the gills by the concurrent action of the opercles, hyoid appa- 

 ratus, jaws, and spiracular openings, it will be noticed in living speci- 

 mens that these parts together constitute a much more perfect i)umping 

 apparatus than is usually seen in the branchial structures of fishes. The 

 gills are specialized beyond what is usual, as indicated by the term Lo- 

 pJiohrancliii ; but this is misleading, as the branchial structures are not 

 really tufted, as may be learned by a careful examination; the inferior 

 and superior branchihyal elements of the branchial skeleton are want- 

 ing, according to Cope, and the arches, to the number of four, appear 

 to be less strongly developed than in other young fishes of the same 

 relative age. I can discern but four pairs, as in the adults, in my spec- 

 imen. The bend downwards of the head involves a bend in the axial 

 structures in the neck. Here the notochord is strongly bent ujion itself 

 as the embryo studied by me clearly shows. The spinal cord also bends 

 sharply downwards just behind the medulla oblongata, as necessitated 

 by the sharp bend in the notochord below it. These constitute some of 

 the most salient differences of Hippocampus as compared with other 

 types of fishes. The skeletal and anatomical characters which distin- 

 guished the Lophobranchs as an order, are given in the following words 

 by Owen (Anat. Vertebrates, I, 12): " Endoskeleton partially ossified, 

 without ribs; exoskeleton ganoid ; gills tufted; opercular aperture small; 

 swim-bladder without air duct. Males marsupial." Cope* defines the 

 order as follows: "Mouth bounded by the premaxillary above; post- 

 temporal simple, coossified with the cranium. Basis cranii simple* 

 Pectoral fins with elevated basis ; well-developed interclavicles. Ante- 



* Report of State Commissioners of Fisheries of Pennsylvania for 1879-80, p. 118. 



1881. 



