to 
periphery of the suctorial disc can be seen the two concentric series 
of presumably sensory processes described in the definition of the genus. 
The outer series consists of a relatively few simple irregularly-spaced 
fingerlike processes; the inner series consists of numerous leaf-like 
structures “frayed”? on their outer edges. One of these is shown in 
fig. 9. At each side of the mouth cavity shown in figure 8 can be 
seen (cush.) a fleshy cushion—the “ Polster” of PLATE. 
The features of the Velasia stage (fig. 10) have already been 
described. 
The Skeleton. 
The greater part of the skeleton of Geotria australis is very similar 
to that of Petromyzon, as reference to figure 11 will show. The prin- 
cipal differences are to be found in connection with the piston (P.C.) 
and median ventral (M.V.) cartilages. Before describing these, some 
minor features appertaining to other cartilages may be mentioned. The 
cartilage of the base of the cranium extends for a short distance on to 
the ventral side of the notochord, as shown in figure 12. The anterior 
dorsal cartilage (fig. 18) is quite hollow (fig. 14) and on transference 
from water to spirit swells out greatly. Posteriorly on each side it 
bears a strongly marked pedicle. The posterior dorsal cartilage (P.D.) 
is of the shape shown in figure 12. The piston cartilage (P.C.) is of the 
form shown in figures 15—17. As shown, it consists of the usual long 
blade-like portion extending posteriorly in the mid-ventral line, the 
dorsal border of the blade being relatively broad (fig. 17). Anteriorly 
it bears in front the two prominent conical pointed horny ‘ teeth” 
(C.P.T.) which arise from a common base, and behind the large thick 
cartilaginous process (D.P.P.C.) which, projecting upwards and back- 
wards, bears hindermost on its dorsal aspect the two flattened “teeth” 
(F.P.T.) and immediately in front of these two curious spherical pads 
(P.O.) which like the teeth are closely apposed in the median line. 
The flattened teeth are convex on their outer sides and coarsely 
serrated on their inner. The pads are gristly in consistency and must 
act as fuleral points (possibly pressing against the hollow anterior 
dorsal cartilage) whilst the tearing conical teeth are in action. Beneath 
the anterior end of the piston cartilage and in close contact with it 
lies the median ventral cartilage—a transversely - situated massive 
skeletal element, broadly grooved on its upper surface for the recep- 
tion of the anterior end of the piston cartilage. The median ventral 
9* 
