The pubic consists of paired bone, to which tarsal bones supporting the fin-rays are attached.
In the Lepidosteoidei the vertebræ are completely ossified, and opisthocœlous, having a convexity in front and a concavity behind, as in some Amphibians. Though the end of the body externally appears nearly diphycercal, the termination of the vertebral column is, in fact, distinctly heterocercal (Fig. [40]). Its extremity remains cartilaginous, is turned upwards, and lies immediately below the scutes which cover the upper margin of the caudal fin. It is preceded by a few rudimentary vertebræ which gradually pass into the fully developed normal vertebræ. The caudal fin is suspended from hæmapophyses only, and does not extend to the neural side of the vertebral column. The neural arches coalesce with the centrum; interneurals simple. The abdominal vertebræ have parapophyses, to which the ribs are attached. Only the caudal vertebræ have hæmal spines.
Fig. 40.—Heterocercal Tail of Lepidosteus.
n, Vertebral column; h, hæmal spines; dn, fulcra; dh, lower fulcra.
In the skull of Lepidosteus the cartilage of the endocranium is still more replaced by ossifications than in Polypterus; those ossifications, moreover, being represented by a greater number of discrete bones; especially the membrane-bones are greatly multiplied: the occipital, for instance, consists of three pieces; the vomer is double as in Polypterus; the maxillary consists of a series of pieces firmly united by suture. The symplectic reaches the lower jaw, so that the articulary is provided with a double joint, viz. for the symplectic and quadrate; the component parts of the lower jaw are as numerous as in reptiles, a dentary, splenial, articulary, angular, supra-angular, and coronary being distinct. The sides of the head are covered with numerous bones, and a præoperculum is developed in front of the gill-cover which, again, consists of an operculum and sub-operculum.
Each hyoid consists of three pieces, of which the middle is the longest, the upper bearing the largest of the three branchiostegals which Lepidosteus possesses; a long and large glossohyal is intercalated between the lower ends of the hyoids. There are five branchial arches, the hindmost of which is modified into a lower pharyngeal; upper pharyngeals are likewise present as in the majority of Teleosteous fishes. No gular plate.
Of the scapulary arch the two halves are separated by a suture in the median line; the membrane-bones are well developed, only a remnant of the primordial cartilage remaining; the supraclavicle is very similar to that of Teleosteous fishes, less so the post-temporal. The base to which the limb is attached is a single osseous plate, supporting on its posterior margin semi-ossified rods in small number, which bear the pectoral rays.
The pubic consists of paired bone, the anterior ends of which overlap each other, the extremity of the right pubis being dorsad to that of the left. The elements representing a tarsus are quite rudimentary and reduced in number (two or three).
The vertebral column of the Amioidei shows unmistakable characters of the Palæichthyic type. The arrangement of its component parts is extremely simple. The centra of the amphicœlous vertebræ are well ossified, but the neural and hæmal arches do not coalesce with the centra, from which they are separated by a thin layer of cartilage. Singularly, not every vertebra has apophyses: in the caudal portion of Amia the vertebræ are alternately provided with them and lack them. The heterocercal condition of the spinous column is well marked: as in the other Holostei the hindmost vertebræ are turned upwards, become smaller and smaller in size, and lose their neural arches, the hæmals remaining developed to the end. Finally, the column terminates in a thin cartilaginous band, which is received between the lateral halves of the fifth or sixth upper caudal ray. Interneurals and interhæmals simple. Only the abdominal vertebræ have parapophyses, with which the ribs are articulated.