1887] The Dipnoan Brain. 545 
birds, and reptiles such an interruption for plexal intrusion, when 
it exists, extends caudad from the porta or lateral orifice of the 
aula, and is known as the rima (“great transverse fissure” of 
anthropotomy). In Ceratodus alone, so far as known to the 
writer, is there a frerima,—that is, a rima extending cephalad 
from the margin of the porta. The brain examined by Huxley 
was evidently ill preserved; the supraplex was mistaken for a 
tela vasculosa (the writer’s audatela). Finally, it would appear 
that the margins of the rima on each side, after the supraplex 
was pulled out, were supposed to be artificial, so as to lead to 
the supposition that the dorsal portions of the cavities of the 
lateral lobes formed a single large “ ventriculus communis.” In 
the writer’s specimens the lobes are separated as high as the 
plexus by a firm, membranous guast-falx, and the prosencephalic 
region of the cranial floor presents a distinct mesal ridge, which 
is absent in Protopterus. In Ceratodus the olfactory lobes are 
_pedunculated instead of sessile, as in Protopterus; but in both 
genera (and apparently also in Lepidosiren) they lie in the plane 
of the general brain-axis, and the proper cerebral outgrowths 
are ventral in position instead of dorsal, as in the Amphibia, 
Reptiles, Birds, and Mammals. Among other features not be- 
fore recorded of Ceratodus is the precommissure and a thick 
valvula reaching more than half-way to the floor of the en- 
cephaloceele (general cavity of the brain). The conarium is 
very large and saccular, and closely attached to the supraplex. 
As stated by Huxley, the tip of the conarium is lodged in a 
distinct depression (the conarial fossa) in the roof of the cranial 
cavity, and the mesencephal does not present any marked fur- 
row between paired optic lobes. Of the resemblances from 
which Huxley concludes that, “ in its cerebral [encephalic] char- 
acters, Ceratodus occupies a central place in the class Pisces” 
[Ichthyopsida, excluding Amphibia], some are trivial, others 
apply to more than one group, and others are founded upon 
errors of observation or interpretation. So far as the brain is 
concerned, Ceratodus has no néar affinity with the Plagiostomes, 
much less with the Holocephals, Ganoids, Teleosts or Marsipo- 
branchs. In the writer’s opinion, the Dipnoi form a class co- 
ordinate with the Amphibia, with which, on the whole, they are 
most nearly allied. The heart needs further study, and the 
pa is unknown. 
