ELASMOBRANCHS. 
1067 
shaped disk of cartilage (fig. 295, prp) on each side 
of the pelvis, armed with sharp teeth at the outer 
(lower) margin, and analogous in position to the prm- 
pubis of the Batrachians and Reptiles. 
The fin-rays of most of the Elasmobranchs are 
cartilaginous, distal sections (joints) of the radialia; but 
in the membrane of the outer parts of the fins — some- 
times, as in the dorsal fins of the Rays, also between 
the tops of the true rays — are set fibrillar or corneous 
rays (figs. 294 — 297, fb), such as we have seen above 
in the embryonic fins of the Teleosts or in the so- 
called adipose fin. In the Sharks and Chinneras similar 
rays support the greater part of the fins (outside the 
radialia). Well-marked calcifications indeed appear, 
under the form of fin-rays almost exactly resembling 
in their exterior the spinous rays we have seen among 
the preceding fishes. Such rays are set in many Elas- 
mobranchs before the unpaired fins (fig. 294, iktd ); 
and in some Rays they arm the tail with mobile wea- 
pons of offence and defence. In palaeontology they bear 
the name of ichtliyodorulites. Agassiz has shown", that 
both as regards their attachment, which is without true 
articulation, and their texture, which exactly corre- 
sponds to the dentinal structure of dermal spines (pla- 
coid scales) and jaw-teeth, these spines are widely dif- 
ferent from the spinous rays of the Teleosts. They are 
mere dermal growths; but inasmuch as they immediate- 
ly overlay and cover with their bases the anterior 
margin of the supporting cartilages of the fins — e. g. 
the spines of the dorsal fin in the Sharks and Chimse- 
ras — they should also be interpreted as most nearly 
representing skeletal tegumentary bones, although they 
are without true ossification. 
The skeleton of the Elasmobranchs is characterized 
by this very want of endoskeletal and tegumentary bone. 
Calcifications of great extent appear it is true, in old 
specimens, under the form af' a thin coat outside or 
just within the surface of the skeletal cartilage, and in 
several Sharks the intercellular mass of the vertebral 
cartilages is radially or (at the centre of the vertebra) 
concentrically calcified. But there is no true ossification. 
The structure of the spinal column, composed of very 
numerous vertebrae, displays the most sweeping altera- 
tions and the most radical divergencies. In the Chi- 
maeras the notochord (fig. 294, ch) remains almost un- 
altered throughout the life of the fish; but around it 
slender calcific rings develop, several, five or even more, 
to each vertebra, whose area may be defined as the space 
between the origins of two pairs of roots belonging to 
the medullary nerves. In the Sharks and Rays, on the 
other hand, the notochord is more or less constricted 
(fig. 296, c), or even snipped off entirely at each ver- 
tebra (fig. 297, c), by the formation of the hollow 
double cones, contiguous at the tops, of which the body 
of the vertebra is composed. The neural arches of each 
vertebra are usually broken up into two or more pairs 
of triangular disks, the true neurapophyses (figs. 294, 
296, and 297, na) applying their base to the body of 
the vertebra, and the other disks, the so-called upper 
intercalaria or intercruralia (in), wedging the apex of 
their triangle in a downward direction between the 
neurapophyses. The spinal canal is sometimes (fig. 294, 
sp and fig. 297, partly between le) closed above by 
the development of unpaired terminal parts, answering 
to the upper spinous processes of more developed ver- 
tebra), and where the vertical fins appear, vertical disks of 
cartilage sometimes afford high supports to these fins and 
their radialia — sometimes produced by the coalescence 
of the spinous processes, as in the Chinneras (fig. 294,5), 
sometimes corresponding to the interspinal or interneural 
bones of the Teleosts. In other cases this closure is 
accomplished by the intercalaria (fig. 296 and fig. 297, 
partly between le). Or the covering may consist simply 
of a fibrous membrane. The neural arches sometimes 
(e. g. in the Rays) grow downwards along the sides of 
the vertebra) or even to their ventral side; but as a 
rule haemal arches (figs. 296 and 297, ha), lower inter- 
calaria (ih), and lower terminal parts (spinous process- 
es) are developed in a downward direction, their extent 
and alterations being similar to those we have just re- 
marked in the corresponding parts of the neural canal. 
In the anterior part of the body, behind the head, both 
the vertebra) and their apophyses often coalesce into a 
continuous mass of cartilage, pierced only by the spinal 
cord and the spinal nerves; and in this part ribs are 
wanting or replaced by a continuous fibrous membrane, 
whereas costal cartilages are present further back. Such 
is the case in the Rays. In the Sharks, on the other 
hand, the vertebra' are more regularly differentiated 
throughout the spinal column; and ribs, though some- 
times wanting, are more frequently present throughout 
the abdominal region. The Holocepliali (fig. 294) show 
Poiss. Foss., tom. Ill, pp. 1 and 212. 
