II. Studies in New-Zealand Ichthyology.–I. On the Skeleton of Regalecus argenteus.

Authors


SUMMARY.

The following are the most important osteological peculiarities of Regalecus argenteus:—

  • 1Owing to the large size of the eyes as compared with that of the cranium, the orbit is enlarged by the formation of a subcranial crest (Pl IV. figs. 7, 10, 11) formed by a downward prolongation of the basis cranii and having a triradiate horizontal section (p. 7, also vide infra5, 9, and 17).
  • 2There is no supraoccipital crest, nor any epiotic or parotic processes (p. 7, figs. 7, 8, and 11).
  • 3A large part of the cranium remains cartilaginous, there being a well-developed and very thick tegmen cranii (figs. 8, 11, and 12, t. cr) quite uncovered by bone, and a large prenasal rostrum (p. n); a considerable part of the auditory capsule also remains unossified (pp. 8 and 9).
  • 4The perichondium lining the cranium is so thick as quite to alter the shape of the cavity (p. 9, figs. 11 and 12).
  • 5The basioccipital (figs. 7–13, b. o) does not enter into the formation of the foramen magnum, being covered above by the united exoccipitals (e. o): it furnishes a medioventral facet to the occipital condyle (o. c2), and is produced downwards into a median vertical plate, which forms the postero-superior portion of the subcranial crest (p. 9).
  • 6The exoccipitals (e. o) unite with one another below, thus forming the ventral as well as the lateral boundary of the foramen magnum and the posterior third of the basis cranii (pp. 9 and 10); they also furnish the supero-lateral facets of the occipital condyle (o. c1).
  • 7The supraoccipital (s. o) is displaced from its usual position by the union with one another in the middle line of the unusually large epiotis (ep. o) (p. 10).
  • 8The prootics (pr. o) are very small, and do not unite with one another below, or help to enclose the canal for the ocular muscles (pp. 10 and 11).
  • 9The opisthotics (op. o) are very large and unite with one another mid-ventrally, forming the anterior two thirds of the basis cranii; each sends off several processes, one of which (op. o2) articulates with the alisphenoid (al. s), and another (o. po4) forms with its fellow the anterior or double portion of the subcranial crest, the wedge-shaped space between the two forming the canal for the ocular muscles (p. 10).
  • 10The sphenotics (sp. o) send off descending processes, which, uniting with ascending processes of the parasphenoid (p. as), form a pair of postorbital pillars (p. or. p), immediately in front of the descending processes of the opisthotics (p. 11).
  • 11Owing to the union of the large alisphenoid (als) with the opisthotic, the fifth nerve apparently makes its exit, partly in front of the alisphenoid (through the foramen marked A), partly through a foramen (V3) in the opisthotic (pp. 11 and 12).
  • 12There are large orbitosphenoids (o. s) uniting with one another in the middle line below, forming a false floor to the anterior part of the brain-case (p. 12).
  • 13There is no trace of either basisphenoid or presphenoid.
  • 14There is an irregular mesethmoid (m. eth) enclosing a small mesonasal cavity (m. n. c) (p. 12).
  • 15The parietals (pa) form faint elevated longitudinal crests on the dorsal surface of the cranium (p. 13).
  • 16The frontals (fr) do not unite with one another in the middle line; they furnish orbital processes (fr1) which roof the orbit; in front they are produced into large, irregular, pointed processes (p. 13).
  • 17The parasphenoid (pa. s) is carried far below the proper level of the basis cranii by the formation of the subcranial crest of which it forms the ventral portion (p. 13).
  • 18The vomer (vo) sends off paired ascending processes which clasp the prenasal cartilage laterally (p. 13).
  • 19There are distinct, loosely attached nasals (fig. 6, na), bounding the nostril above (p. 13).
  • 20The suborbital chain is reduced to two preorbital bones (fig. 6, p. or), the anterior of which bounds the nostril below (pp. 13 and 14).
  • 21The mesopterygoid (fig. 6, 14, and 15, ms. pt) apparently begins as a parostosis, the ossification afterwards extending into the pterygoid cartilage (p. 14).
  • 22The premaxillæ (fig. 6, p. mx) have large nasal processes (p. mx1) which embrace splint-wise a large laterally compressed cartilage, which works in a median groove of the tegmen cranii and allows for the protrusion and retraction of the jaws (p. 15).
  • 23The mandible (figs. 6 and 14) is remarkably short and high (p. 15).
  • 2The first and second branchial arches (fig. 17) are terminated dorsally by a segment apparently not before described (pa. br1, pa. br2), and which it is proposed to call the parabranchial (p. 17).
  • 25The copulæ, or medio-ventral elements, of the branchial skeleton are eight in number (fig. 17, cp.1-cp.8), of which the first (cp.1) is the entoglossal or basihyal; of the others, two (cp.3 and cp.5) join the ventral ends of the first and second arches respectively, and are considered to be the only two true basibranchials; the others are intermediate, each between two arches, and it is therefore proposed to call them interbranchials (p. 18).
  • 26The vertebral column (fig. 5) consists of ninety-three vertebræ, with the neural arches of which are connected two hundred and six interspinous bones, serving for the attachment of two hundred and five (15+190) dermal fin-rays.
  • 27There is a gradual increase in length of the vertebral bodies from before backwards, the first (fig. 20, c. 1) being inch, the ninety-second (fig. 25, c. 92) 4 inches in antero-posterior dimensions (p. 20).
  • 28The atlas bears long transverse processes; the second vertebra is devoid of these, which, however, reappear in the third (p. 20).
  • 29Small slender ribs are borne by the eighth to the twenty-fifth vertebra inclusive (p. 21).
  • 30None of the hæmal processes unite below, so that there are neither hæmal arches nor hæmal spines in any part of the vertebral column (p. 22).
  • 31The bone terminating the vertebral column posteriorly (PI. VI. fig. 25, c 93) is considered to be a demivertebra, that is to represent the anterior half of a vertebral body; it is in the same straight line with the rest of the vertebral column, so that the tail is strictly diphycercal (pp. 22 and 23).
  • 32In the anterior part of the body there is an average of two, in the posterior part, of three interspinous bones to a vertebra (p. 23).
  • 33The fin-rays are articulated to the interspinous bones not directly, but through the intermediation of ovoid cartilaginous nodules (figs. 20 and 25, c. n), which alternate with the interspinous bones and are considered to represent a second or distal set of radials or pterygiophores (p. 24).
  • 34The anterior nine interspinous bones lie altogether in front of the atlas and overhang the skull (fig. 20); the first five of them are peculiarly modified and ankylosed together to support the crest (p. 25).
  • 35The posttemporal (PI. IV. figs. 6, 8, and 10, and PI. V. fig. 18, p. tm) is not forked, and is closely applied to the dorsal surface of the skull, in a groove between the epiotic and the pterotic (p. 26).
  • 36There are three brachials (figs. 6 and 18, br.1–3) supporting the pectoral fin; they are irregular flat bones, having a vertical height equal to that of the scapula (p. 26).
  • 37In front of the first well-developed ray of the pectoral fin (fig. 6, pc. f) is a small bone which probably represents a rudimentary first (preaxial) ray (p. 26).
  • 38The innominate, or pelvic bones (figs. 6 and 19, o. in), are very large, and unite with one another by a double symphysis (p. 27).

Ancillary