1877. ] 41 (Cope. 
PISCES. 
HYOPOMATA. 
It has sometimes appeared to the writer that a modification might with 
advantage be introduced into the system of Fishes, as left in his synopsis of 
the osteology of the subject, published in the Transactions of the American 
Philosophical Society, 1870, p. 449, and the Proc. Amer. Ass. Adv. Science, 
1871, p. 826. The sub-classes of fishes there recognized were five ; viz. the 
Holocephali, the Selachii, the Dipnot, the Crossopterygia, and the Actinop- 
tert. As it appears that the structural differences existing between the last 
two divisions are not so great as those which distinguish the others, it is 
proposed to combine them into a single sub-class, to be called the Oper- 
culata. The definitions of the four sub-classes will then be as follows : 
I. Suspensorium continuous with the cartilaginous cran- 
ium, with no hyomandibular nor rudimental opercular bone ; 
no maxillary arch; pelvic bones present; axial series of fore 
limb shortened, the derivative radii sessile on the basal 
pieces ; axial series of hinder limb prolonged in (......... Holocephali. 
Il. Suspensorium articulated with the cranium; no 
maxillary arch; no opercular nor pelvic bones; bones of 
HATE PPeA SUN LOH AS trey Srecws eh ent Neue ok ave loyal e ieistei/sieiave @ eteveiel cessayysiateiate Selachii. 
III. Suspensorium rudimental, articulated with cranium, 
supporting one or more opercular bones ; cranium with su- 
perior membrane bones; no maxillary arch ; a median pel- 
vic element ; the limbs supported by segmented unmodified 
GDSBkk Avon Sou ded S06 bod dU COC DO ODE AED OnE e RODD duu cco dn one Dipnoit. 
IV. Hyomandibular and palatoquadrate bones articulating 
with cranium, supporting opercular bones ; a maxillary arch ; 
no pelvic element ; axes of the limbs shortened, the derivative 
radii sessile on the basal pieces... 0.0... ccccecccccssseccsee Hyopomata. 
The primary divisions of the Hyopomata are indicated by the structure 
of the fins, of which there are three principal modifications, as follows : 
A. Derivative radii present in both limbs ; in the anterior 
supported by an axial segment with one or more basal or 
derivative radii, forming a peduncle ; in the hind limbs the 
derivative radii sessile on axial segment only....-........ Crossopterygia. 
B. Derivative radii few in the fore limb, sessile on scapu- 
la ; present in hind limb, and sessile on axial segment.... Chrondostet. 
C. Derivative radii few in the fore limb, sessile on the 
scapula ; wanting or very few and rudimental on the hind 
limb so that the dermal radii rest on the axial element.... Actinoptert. 
The classification of the Actinopter? then continues as in the memoirs 
above quoted. 
PROG. AMER. PHILOS. soc. xviI. 100. F 
