12 BULLETIN OF THE 
with generic diagnoses, is subject to important modifications on account 
of individual and specific variation. A specimen of ZH. peetorosus before 
me has the first and second fused with the hypobranchials, and longi- 
tudinally divided or partly divided through the middle, the third fused 
with one of the hypobranchials, the fourth distinct but divided through 
the middle, and the fifth normal. A specimen of H. maculatus has the 
first appearing as a small lozenge-shaped lump, the second fused with the 
right hypobranchial, the third fused with both of the corresponding pair 
of hypos, and the fourth either suppressed or fused and divided. 
Hypobranchials (h-br). — Anteriorly these cartilages are distinct and 
moderately slender; posteriorly they become mere processes on the 
sides of the basibranchials. As the articulation with the ceratobran- 
chial takes place on the upper side of the posterior lobe of the inner 
extremity of the preceding ceratobranchial, each hypo- is really articu- 
lated with two ceratobranchials. 
Ceratobranchials (c-br). —These are slender and long. The first pair 
articulate with the posterior angles of the basihyal, hypohyals. Each 
succeeding articulates with the hypobranchials on the upper side of the 
posterior lobe of the broadened inner extremity of the one immediately 
preceding. The sixth pair is much stouter, articulates directly with the 
last basibranchial, without hypos, and bears a downward-inflated margin 
on its outer extremity. 
The epibranchials (e-br) are slender. The upper ends are thicker and 
broader and the pharyngobranchials articulate against the outer side. 
The pharyngobranchials (p-br) are slender and elongate, with the ex- 
ception of the sixth pair. The latter are short and flexible; they are 
fused with the epibranchials. 
The Vertebral Column. 
Plate X. 
The notochord is persistent. For a short distance back of the head 
there are vertebral constrictions ; this condition only obtains in the few 
vertebree that are somewhat calcified(fig. 3). Here the condition is sim- 
ilar to that in Centrophorus, as figured by Kolliker, or in the posterior 
vertebree of Heptabranchias. Behind this, in the much greater portion 
of the column, the vertebre are much less distinct, and the notochord 
maintains a uniform diameter (fig. 2). Forward the vertebrze can be 
distinguished with readiness, but in the middle of the body, though ex- 
ternally the different segments are well marked (fig. 1), a longitudinal 
