1384,] 585 (Cope. 
notochordal sturgeons as descendants of Crossopterygia, whose modern 
representatives are osseous. The primitive Crossopterygia, and probably 
even the Actinopteri, were doubtless as cartilaginous as are the existing 
sturgeons : 
( Actinopteri. 
Hyopomata = Chondrostei. Batrachia. 
( Crossoptery gia. pe 
Blasmobranchi <=) L¢hthyotomi. Dipnoi. 
¢ Selachii. orn 
i TLolocephali. 7 
In this phylogeny, the Holocephali, which have not differentiated a 
suspensorium, are regarded as the primitive fishes, although the living 
representatives display some specialized characters, as, for instance, a 
membranous gill-cover which conceals the primitive slits. The line to 
the right continues the monimostylic character and passes into the reptiles, 
whose primitive types are also monimostylic, as Johannes Miiller called 
them. In the later forms or streptostylicate reptiles of Miller (Lacertilia, 
Ophidia), the quadrate becomes freely articulated.* 
In the left hand series, the Elasmobranchs immediately present us with 
the free suspensorium or hyomandibular, which is a well-known character 
of the remainder of the line, the modifications being the addition of sepa- 
rate elements, as the metapterygoid, ‘‘quadrate,’’ and symplectic. 
The penetration of ossification into the chondrocranium of Didymodus, 
in regions not ossified in either fishes or batrachia (sphenoid and pre- 
sphenoid), and into regions not ossified in any vertebrate (frontal and 
pariétal cartilages), may be, so to speak, only a local phenomenon, and 
not indicative of extensive phylogenetic consequences. For if it be so 
regarded, it evidently proves too much, giving affinities in the base of the 
skull to the reptiles, and in the roof exhibiting a character more highly 
developed than any known form of vertebrata. 
The Ichthyotomi include, so far as yet known, but one family, the Hybo- 
dontide of Agassiz. According to that author this family includes four 
genera, Hybodus, Pleuracanthus, Cladodus and Sphenonchus. It ranges 
from the coal-measures to the Jura inclusive. 
The genus Didymodus may be described as follows : 
Frontal plane well defined on each side by the temporal fosse, and ter- 
minating in two cornua posteriorly. Anterior nares on the superior gur- 
face of the muzzle. Supraorbital (or nasal) bones well separated on the 
median line and constituting the only membrane ossification. Teeth with 
large lateral denticles. 
The species Didymodus compressus Newberry, may be defined as follows : 
Skull with massive walls. Form elongate, depressed, the orbit not ex- 
*The phylogeny of the Reptilian series can be found in the Proceedings 
American Association Advancement of Science, xix, 1871, p. 233, The Batrachia 
are supposed to be their ancestors. 
