﻿HOLOPTYCHIID^, 321 



Holoptychiidre the sensory canal-system seems to form merely 

 grooves in the exoskeleton ; while in the Rhizodontida) and Osteo- 

 lepidae it usually perforates the bones, and is especially conspicuous 

 upon parts of the head from the series of dot-like apertures by 

 which the closed canals open externally. 



The notochord seems to have been always more or less persistent, 

 but the cartilages of the arches are at least superficially calcified, and 

 in the more specialized genera there occur robust, closely arranged 

 ring-vertebrse. 



The pectoral arch exhibits two well-developed pairs of membrane - 

 bones — a large clavicle and a smaller infraclavicle, sometimes very 

 firmly united by an upward process of the latter. A supraclavicular 

 element has also been observed, but there is no definite information 

 as to its precise characters. The lobe of the paired fins is supported 

 by endoskeletal cartilage, arranged on the plan termed archiptery- 

 gial by Gegenbaur : and it is interesting to note that even in the 

 short, obtusely lobate fins, the axis is merely shortened and the 

 parameres of one side somewhat atrophied, while those of the other 

 side are enlarged. There is thus no dibasal or tribasal arrangement 

 of the cartilages such as characterizes the pectoral fins of Polypterus. 



In the median fins, the rays are always delicate and very 

 numerous, overlapping the ends of the supporting cartilages, which 

 are robust and comparatively few in number. The dorsal and anal 

 fins always exhibit more or less lobation, and are supported by two 

 series of cartilages, the proximal conveniently termed axonosts, and 

 the distal baseosts. There is but a single, club-shaped axonost to 

 each of the fins, the broad distal end of this element bearing about 

 three to six elongated, rod-like baseosts, which are sometimes 

 jointed at intervals and bifurcating. The arrangement of the 

 supports of the caudal fin is not clearly ascertained. 



Family HOLOPTYCHIID^]. 



Body fusiform, with cycloidal, deeply-overlapping scales, more or 

 less enamelled. Head and opercular apparatus with well- developed 

 membrane-bones : parietals large and separate : frontals separate, 

 not fused into a continuous plate with the adjoining elements ; no 

 parietal or frontal foramen ; interoperculum absent ; jugular plates 

 comprising one large pair, flanked on either side by a lateral series. 

 Dentary bone of mandible thin and deep, bearing a series of small 

 teeth, and with well-developed infradentaries, much bent inwards 

 below ; an inner series of few, large, broad, shuttle-shaped bones, 



PART II. T 



