60 
WELLBURN I ON THE GENUS MEGALICHTHYS, AGASSIZ. 
Head. — Internal anatomy in M. (ectosteorhachis) ciceronius 
Cope,* the chondrocranium is in some degree ossified, and the 
parachordal cartilages are ossified to form two siibtriangular bones 
which present one angle forwards, and having the internal side 
Avhich bounds the chordal groove straight and longitudinally 
grooved. The antero-external side is oblique and nearly straight, 
and is overhung by the cranial roof. These ossifications embrace 
the chorda dorsalis posteriorly, and are continued a short dis- 
tance posteriorly as a tube. Anteriorily the chordal grove is 
open, and we here get a good illustration of a permanent 
embryonic type (Cope, opus cit.). 
According to Dr. Young (see Quart. Journ. Geol. Soc, Yol. 22 
(1866), p. 605) the basilar region is well ossified and includes a 
massive basioccipital which projects behind the vertical posterior 
wall of the cranium and sometimes has its length increased by 
coalescence with at least the first vertebral ring, whose neural 
process remains distinct. In a lateral view, the aliophenoides (?) 
and an incomplete interorbital septum (?) are well shown. 
The hyomandibular is not shown in any of the specimens 
I have seen, but it is probably (as in Rhizodopsis)j covered by 
the preoperculum, and extends from the squamosal above down- 
w^ards and slightly backwards to the articular extremity of the 
mandible below. 
Cranial Anatomy (PI. XY., Figs. A, B and C). — The whole 
of the cranium is covered with thick dermal plates, which exhibit 
a definite arrangement, and there is a considerable development 
of membrane bones on the roof of the mouth. The shield of the 
cranial roof is divided by a much-pronounced transverse suture into 
two parts. The posterior portion consists chiefly of a pair of long 
narrow bones (Pa.), the parietals, which are divided down the 
middle line by an irregular suture, the bones are twice as wide 
behind as in front, their external margins first run nearly straight 
*Proc. Anier. Phil. Soc, Vol. 20, page 628. 1883. 
t Trans. Roy. Soc. of Edinburgh, Yol. XXX., page 171 (Traquair). 
