GLOSSARY OF TECHNICAL TERMS CXXXV 



Notochord. The embryonic cartilaginous vertebral column, persistent in lampreys, sharks, and 

 rays, and most ganoids. 



Nuchal. Pertaining to the nape. 



Obsolete. All but disappeared; only faintly apparent. 



Occiput. The back of the head. 



Ocellus. An eye-like spot. 



Opercle. The gill-cover. (Fig. 1, o. See operculum.) 



Operculum. A bone of the side of the head, forming the major portion of the covering of the 

 gill cavity. 



Opercular flap. A backward prolongation of the posterior angle of the opercle. (Fig. 62, 63.) 



Opercular gill. A rudimentary gill on the lower inner face of the operculum in gars and stur- 

 geons. It is a true gill, receiving venous blood, in which respect it differs from a pseudo- 

 branch. In the gars, in which there is both an opercular gill and an exposed pseudo- 

 branch, meeting at an angle on the inner face of the operculum, the opercular gill may be 

 recognized by its inferior position and by the downward and backward direction of its gill- 

 filaments. (See pseudobranch.) 



Opisthoccelian. Concave behind only; said of the vertebrae of gars, which connect by ball and 

 socket joints, as in reptiles. 



Orbit. The bony eye-socket. 



Ossicula auditus. (See Weberian ossicles.) 



Palatine. A paired bone of the roof of the mouth. (Fig. 56, pi.) 



Papilla. A small fleshy projection. 



Papillose. Covered with papillae. 



Parietal. One of the roofing bones of the skull. 



Pectinate. Having teeth like a comb. 



Pectoral. Pertaining to the breast. 



Pectoral arch. (See shoulder girdle.) 



Pectoral fins. The anterior or uppermost of the paired fins. (Fig. 1, p.) 



Pectoral girdle. (See shoulder girdle.) 



Pelvic arch, or girdle. The bones to which the ventral fins are attached; pubic bones. 



Peritoneum. The membranous inner lining of the abdominal cavity. 



Pharyngeal bones. Bones representing a fifth gill-arch, behind the gills, opposed to each other, 

 usually in several upper and one lower pairs, as masticatory structures, for which purpose 

 they are, as a rule, armed with teeth. (Fig. 57, Iph. and Fig. 56, uph.) 



Physostomous. Raving the air-bladder connected with the oesophagus by an open duct. 



Plectospondylous. Having the anterior vertebrae modified and furnished with Weberian ossicles. 

 (See Weberian ossicles.) 



Plicate. With wrinkle-like folds. 



Postclavicle. An element of the shoulder girdle. 



Postfrontal. A roofing bone of the skull. 



Post-temporal. The element of the shoulder girdle which connects it with the skull. 



Prefrontal. An anterior roofing bone of the skull. 



Premaxillary. The paired bone forming the front of the upper jaw. (Fig. 1, pmx.) 



Preoperde. A bone of the cheek. (Fig. 1, po.) 



Preorbital. A large bone lying in front of the eye. 



Procurrent. Coming forward. Said of small accessory caudal rays encroaching on the caudal 

 peduncle in front of the base of the caudal fin. 



Protractile. Capable of being drawn forward. Said of premaxillaries which are extensible for- 

 ward and are separated (when retracted) from the skin of the forehead by a groove. 



Pseudobranch. A rudimentary gill-like structure, not functioning as a gill, developed on the 

 upper inner side of the opercle, differing from true gills in the fact that it is supplied with 

 arterial rather than venous blood. The pseudobranch may be exposed in the branchial 

 cavity (as in the perch) or covered entirely by skin or hidden in the spiracular cavity (as 

 in sturgeons and the paddle-fish). (See spiracle and opercular gill.) 



