HOLOPTYCHIIDiE, 
321 
Holoptychiidse the sensory canal-system seems to form merely 
grooves in the exoskeleton; while in the E-hizodontidm and Osteo- 
lepidse 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-vertebrce. 
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 alwaj’s 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 HOLOPTYCHIIDrE. 
Tody 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. v 
