1884. | 579 [Cope. 
brain. But it is singular that no one, so far as I know, has insisted upon 
the fact, not only that the Teleostean brain is essentially similar to that of 
the Ganoids, but that it is exactly in those respects in which the Ganoids 
and Teleostei agree in cerebral structure that they differ most markedly 
from the Plagiostomi and Chimeeroidei. 
“With respect to the chiasma of the optic nerves, the exact nature of 
that structure has not yet been properly elucidated either in the Selachians 
or in the Ganoids. But, whatever may come of such an investigation, 
the establishment of the existence of a true chiasma in the Ganoids, and 
of its absence in Teleosteans, can have but little bearing on the question 
of their affinities, since Wiedersheim has shown that a simple decussation 
of the fibres of the optic nerves, as in ordinary Teleosteans, takes place in 
many lizards.’’ 
In 187'7* I proposed the following primary divisions of the fishes, and 
have seen no reason to alter my views as to their value as a correct ex- 
pression of the affinities and diversities of this class of Vertebrata. The 
system differs only from that of 1871 in the consolidation of the Crossop- 
terygia and Actinopteri into a single sub-class, the Hyopomata; and in a 
few corrections of the definitions given. They are as follows : 
I. Suspensorium continuous with the cartilaginous cranium, with no 
hyomandibular, No rudimental opercular bone ; no maxillary arch ; 
pelvic bones present ; axial series of fore limb shortened, the deriva- 
tive radii sessile on the basal pieces; axial series of hinder limb pro- 
LOMMCCIMMMIS iis ise Qe viv vivieeeeCen eset eeneuage cee Holocephali. 
II. Suspensorium articulated with the cranium; no maxillary arch; no 
opercular nor pelvic bones; bones of limbs as in the last. ........... 
Hlasmobranchi. 
III. Suspensorium rudimental, continuous with cranium, supporting one 
or more opercular bones ; cranium with superior membrane bones ; 
no maxillary arch; a median pelvic element ; the limbs supported by 
segmented unmodified aXesss.cvecive sree ces vei yio erin h dosiley eee TLObs 
IV. Hyomandibular and palatoquadrate bones articulated with cranium, 
supporting opercular bones; a maxillary arch; no pelvic element ; 
axes of the limbs shortened, the derivative radii sessile on the basal 
DISCOS ewe eau Ca visihwele aoe ALUN THOME SUN Gut eee cela OD OMTauOn 
Tn the definition of the Dipnoi, it is necessary to make the correction in 
accordance with the best observations on fresh specimens, above referred 
to, as [ have not been able to determine the question from dried speci- 
mens in the Hyrtl collection. The suspensorium cannot be properly said 
to be articulated to the cranium in the sense in which it is said to be such 
in the Elasmobranchi. In the latter it is articulated by ginglymus; in 
* Proceedings of the American Philosophical Society, 1877, p. 25; and in the 
Annual Reports of the Commissioners of Fisheries of Pennsylvania for 1879-80, 
p. 67 and 1881-2, p, 111. 
