854 T^^i^ American Naturalist. [October, 
Dermal cranial ossifications and opercular bones ; 
no claspers ; a maxillary arch ; Teleostomi. 
HOLOCEPHALI. 
But one order of this subclass known. 
A single external branchial fissure; actinotrichia 
present ; basilars, axonosts and neural spines 
articulating with each other ; pectoral fin with 
three axonosts and numerous basilars ; ventrals 
with elongate axonosts and basilars ; Chimceroidei. 
The order Chim.eroidei embraces only the family Chimaeridae. 
DIPNOI. 
There is but one order of this subclass known as yet. 
Actinotrichia ; baseosts and axonosts of median fins 
continuous with neural spines ; paired fins with a 
single basal axonost and numerous segments 
continuous with it ; swim-bladder cellular ; Sirenoidei. 
Three families represent our knowledge of the order Sirenoidei; 
the Dipteridae, the Ceratodontidae, and the Lepidosirenidae. 
ELASMOBRANCHII. 
There are two orders of this subclass. 
A basioccipital and exoccipital elements ; actinotri- 
chia; baseosts and axonosts continuous with 
neural spines ; paired fins with a single basal 
axonost, and numerous others in line with it ; 
claspers simple ; Ichthyotomi. 
No basi or exoccipital ; median baseosts and axonosts 
continuous with vertebral spines ; several axo- 
nosts to paired fins, and numerous baseosts ; 
claspers complex ; actinotrichia ; Selachii. 
To the order Ichthyotomi belong the families Xenacanthidae 
and Cladodontidae. To the Selachii belong the following: 
{Squali) Psammodontidae, Petalodontidae, Cochliodontids, Cestra- 
