a 
1884. ] 581 [Cope. 
reference to the Hlasmobranchi is confirmed by the following characters : 
(1) The nares are not oral. (2) There isa large fontanelle on the summit, 
of the muzzle. (3) There are processes corresponding to the lateral alee 
of the basicranial axis. 
In another character Didymodus differs from this and all other sub-classes 
of the Pisces. This is the penetration of the granular ossification through- 
out the chondrocranium. 
In the following characters it agrees with the Dipnoi: (1) The distinct 
exoccipital, pariétal, and frontal elements. (2) The occipital cotylus. 
(8) The posterior bifurcation of the frontal cartilage. 
In the following characters Didymodus resembles the Hyopomatous or 
true fishes: (1) In the basioccipital bone with condyle. (2) In the ?0s 
intercalare or pteroticum. (8) The presence of a distinct element articu- 
lating with the proximal end of the hyomandibular. (4) The presence of 
membrane bones in the position of frontals. 
The characters above cited as constituting resemblances to the true 
fishes, will not, it appears to me, permit the reference of this genus to any 
of the divisions of sharks established by Prof. Gill. I therefore proposed 
anew order of the Elasmobranchi* for its reception, with the following 
name and definition. . 
A basioccipital bone and condyle. Occipital, ? pterotic, and frontal bones 
distinct. Supraorbital (or nasal) bones present............lehthyotomi. 
The remaining Elasmobranchi, in which the above characters are want- 
ing, may be termed by way of contrast, utilizing an old name, Selachii. 
Were it not for the probable presence of the free hyomandibular bone, 
the order Ichthyotomi might be regarded, in the absence of knowledge of 
its limbs, as the possible ancestor of the Rhachitomons Batrachia. But as 
the Batrachia have no distinct suspensorium, or are, to use Miiller’s con- 
venient term, monimostylic, their origin must still be sought for in some yet 
undiscovered type of Dipnoi. It is on the other hand very probable that 
the Ichthyotomi are the group from which the Hyopomata derived their 
origin. The distinct basioccipital with its two foramina, the superior 
origin of the hyomandibular, and the superior nostrils, all point towards 
the true fishes. The tribe of Hyopomata which must be their most im- 
mediate descendents, are the Crossopterygia, as I define that division. 
I must now compare the Ichthyotomi with such groups of the Hyopo- 
mata as they may be supposed to approach most closely. I begin by refer- 
ring to the marine eels of the order Colocephali. In 1871} I characterized 
this order as follows: ‘‘Pariétals largely in contact; opercular bones 
rudimental ; the preoperculum generally wanting. Pterygoids rudimental 
or wanting ; ethmoid very wide. Symplectic, maxillary, basal branchi- 
hyals, superior and inferior pharyngeal bones, all wanting, except the 
fourth pharyngeal. This is jaw-like, and is supported by a strong supe- 
rior branchihyal ; other superior branchihyals wanting or cartilaginous.”’ 
* American Naturalist, 1884, 413, 
t+ Proceedings American Ass, Adv, Science, xx, pp. 328-334. 
