12 BULLETIN OF THE 



■with generic diagnoses, is subject to important modifications on account 

 of individual and specific variation. A specimen of //. xj^ctorosus before 

 me has the first and second fused with the hypobrancliials, 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 (Ji-hr). — 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. 



Ce7'atobra7ichials (c-br). — These are slender and long. The first pair 

 articulate with the posterior angles of the basiliyal, hypohyals. Each 

 succeeding articulates with the hypobrancliials 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 

 vertebrse that are somewhat calcified (fig. 3). Here the condition is sim- 

 ilar to that in Centrophorus, as figured by Kolliker, or in the posterior 

 vertebra; of Heptabranchias. Behind this, in the much greater portion 

 of the column, tlie vei'tebrse are much less distinct, and the notochord 

 maintains a imiform diameter (fig. 2). Forward the vertebrae 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 



