320 



ICHTHYOLOGY. 



Classifica- is variously and singularly modified in the different Plagio- 

 tion stomous genera. 



Chondrop- j^^ ^j^^ appended arches of the cranium there is a similar 

 J confluence ot cartilage. A smgle strong suspensory pedicel 

 ^ ^ ^ articulated to the lateral posterior or mastoid process gives 

 attachment to the hyoid arch by a close joint, and to the 

 mandibular arches by two ligaments. The maxillary arch 

 is suspended by a ligament passing from its ascending pala- 

 tal process to a notch between the vomerine and the an- 

 terior supracranial cartilage. From this point the upper 

 jaw is continued in one direction forwards and inwards, 

 completing the arch by a ligamentous symphysis with its 

 fellow ; and in the opposite direction to coalesce with the 

 outer side of the tympanic pedicel, and form the more im- 

 mediate articulation f()r the lower jaw or mandibular arch. 

 Each half of the mandible consists of a single cartilage, the 

 two being united together at the symphyses by ligament. 

 Two slender labial cartilages are developed on each side 

 of the maxillary, and one on each side of the mandibular 

 arch. (The parts described in this paragraph, with the teeth 

 which they support, are the " Shark's jaws" usually brought 

 home by seamen.) 



Four or five short cartilaginous rays, in Squadna, diverge 

 from the posterior margin of the tympanic pedicle, and sup- 

 port a membrane answering to the opercular flap of osseous 

 fishes ; these rays are the skeleton of the diverging appen- 

 dage of the tympano-mandibular arch. The hyoid arch in 

 the Squati/ia, as in most other Plagiostomes, consists of 

 two strong lateral cerato-hyoids, and a median flattened 

 basi-hyoid. Short cartilaginous rays extend from the back 

 part of the ceiato-hyoid to support the membranous wall of 

 the branchial sac ; these are the homologues of the bran- 

 chiostegal rays of osseous fishes. 



The CestracioH is not so far advanced in cranial develop- 

 ment as the Sqiiatina, and the cartilaginous basioccipital 

 has a hollow conical articulation with the end of the spine ; 

 but the Skates and Rays have the rkuU moveably articu- 

 lated by two condyles and an intervening space, as in Sc/ua- 

 tina and Zygtena. The skull of the Rays is flat and broad ; 

 the upper wall membranous to a greater or less extent, ex- 

 cept in JSarcine, where it is closed by cartilage. The an- 

 terior or vomerine part forms a long pyraniiiial rostrum, to 

 which are articulated cartilages connecting its extremities 

 with the radial or anterior angles of the pectoral fin, or 

 largely expanded hand ; in the space between the pectorals 

 and skull the Torpedidte carry their electric batteries. The 

 tympanic pedicels are short and thick ; the maxillary and 

 mandibular arches long and wide, stretching across the un- 

 der part of the head. 



The capsules of the organs of sense are all cartilaginous ; 

 that of the ear is involved, as we have said above, in the 

 lateral walls of the cranium ; that of the eye is articulated 

 by a cartilaginous pedicel with the orbit ; and the olfactory 

 sacs are covered by vaults of the nasal processes. The long 

 lateral processes supporting the eye-balls of the "Hammer- 

 headed Sharks" have already been alluded to. In Mylio- 

 hates and Rhinoptera a cartilage is attached to the pro- 

 longed anterior angle of the pectoral fin, connecting it with 

 the internasal cartilage, and supporting a number of branched 

 and jointed cartilaginous rays. A like series of rays on the 

 other side of the lieatl meet these before the skull on the 

 median line, and form what Miiller terms, as we have said 

 in a preceding page, " cranial fins ;" but Owen considers 

 them to be merely dismemberments of the pectoral fins, 

 and thinks that the real cranial fins are the opercular and 

 brancliiostegal appendages of the tympanic and hyoidean 

 arches. 



In tracing the progressive rise of organization of the 

 spinal column in the Cartilaginous fishes, Owen commences 

 with the Myxinoid fishes, whose spine is a gelatinous chorda 

 with fibrous investments. The separation of the upper 



layers of the fibro-niemhranous sheath forms a neural canal, Classifica- 

 and a haemal canal is produced below in like manner. In tion — 

 the Sturgeon there is a continuous central gelatinous chorda, Chondrop- 

 but the inner layer of the fibrous investment assumes the *''■■>'?'*"'• 

 texture of tough hyaline cartilage, while distinct, firm, and ^^'"^ 

 opaque cartilages are developed in the outer layer. Above 

 the coalesced centra these cartilages have a similar arrange- 

 ment to the homologous parts of osseous fishes ; a neural 

 canal is formed by two cartilaginous neurapophyses, each in 

 the young fish formed of two superimposed pieces, and the 

 arch is crowned by a single short neural sjiine. There is a 

 short thick parapophysis on each side, with their bases ex- 

 panding below to form a haemal canal in the tail, by iheir 

 union beneath the centrum. They include a small inter- 

 hfemal cartilage on each side, and towards the end of the 

 tail, haemal cartilaginous spines are developed. Short pleura- 

 po|)hyses or ribs are articulated to the ends of the parapo- 

 physcs. A beautiful progression, in the development of the 

 spinal column and of the proper vertebral structure, is traced 

 out by Miiller in his Anatomie der Myxinoiden, ap.d re- 

 ferred to by Owen, from whom we quote. In the Basking 

 Shark {Selache) the vertebral centra are formed of hollow 

 cones, meeting at their apices in the interior, and having 

 their cups filled with the remains of the gelatinous chorda. 

 This semi-fluid, slightly compressible, and very elastic, gives 

 flexibility to the spine, while it sustains the shocks of violent 

 inflexions. Strength and lightness are imparted to the 

 centra by concentric plates of bone, crossed by others radi- 

 ating like the spokes of a wheel from the apices of the cones. 

 The highest stage of vertebral development occurs among 

 Galei and other Ni/ctitantes or Sharks, which have a nicti- 

 tating eyelid. In them the external surfaces of the centra, 

 and their internal cones, are covered by a smooth osseous 

 crust. A (e\v Sharks only, as (Jarcharias, Hepla nc/tus, and 

 Alopias, have the ribs connected to the centrum at the base 

 of the parapophyses ; in most they are attached, as in the 

 Sturgeon, to the ends of these processes. 



The gills of the Plagiostoini do not present the pecti- 

 nated arrangement of slender tapering processes in a double 

 row, like the osseous fishes, but consist of folds of membrane 

 on plane surfaces. A pseudobranchia coexists with a hyoid 

 uniserial gill in most Plagiostomes, and in those which have 

 spout-holes it is developed on the anterior wall of these out- 

 lets from the branchial cavity. The Plagiostomes have two 

 or more transverse rows of semilunar valves in the bulbus 

 arteriosus, or beginning of the arterial system; Galeus has 

 two rows of three valves ; Sphyrna, Mustelus, Acanlhias, 

 Alopias, Lamna, Ehiiiohates, Torpedo, and Accipenser 

 have three rows of valves ; Hexanihm, Heptanchus, Cen- 

 trophornx, and Tryr/on have four rows of valves; Scymmis, 

 Squatina, Myliobates, and Raia balls, or the common Blue 

 Skate, have five rows. Cephaloptera has a very large 

 bulbus arteriosus, and presents internally three longitudinal 

 angular ridges, at the sides of which are small valves dis- 

 posed in pairs, and in four or five rows ; besides these there 

 are three larger valves at the beginning, and three at the 

 end of the bulb. The optic nerves of the Plagiostomes are 

 connected by a commissure, but do not decussate as in the 

 osseous fishes. And in this order the plexus of arteries, 

 termed a vasoganglion, which lies between the layers of 

 the choroid coat of the eye of most osseous fishes, does not 

 exist. Miiller found that generally when a fish possessed 

 pseudobranchiae, it had also the optic vasoganglia ; but the 

 Plagiostomes, Sturgeon, and Lepidosiren are exceptions, 

 having the pseudobranchiae, but wanting the vasoganglia. 

 Mormyrics, Cobilis, Silurits, Pimelodiis, Syiiodon, Murana, 

 and some other genera of the serpentiform apodals, have 

 neither the pseudobranchias nor the vasoganglia, but in 

 most osseous fishes in which they have been sought for 

 they have been found existing together. 



The only other parts of structure of the Plagiostomes that 



