OSTEOLOGY,] ICHTHYOLOGY 645 type. The vertebral column is represented by a simple chorda dorsalis or notochord only, which extends from one extremity of the fish to the other, and, so far from being expanded into a cranial cavity, is pointed at its anterior end as well as at its posterior. It is enveloped in a simple membrane like the spinal cord and the abdominal organs, FIG. 30. Anterior end of body of Branchiostoma (magn.). d, chorda dorsalis ; e, spinal cord;/, cartilaginous rods; g, eye; h, branchial rods; , labial carti lage ; *, eral cirrlii. and there is no trace of vertebral segments or ribs; a series of short cartilaginous rods, however, above the spine evidently represent apophyses. A maxillary or hyoid apparatus, or elements representing limbs, are entirely absent. The skeleton of the Cydostomata (or Marsipobranchii) (lampreys and sea-hags) shows a considerable advance of development. It consists of a notochord, the anterior pointed end of which is wedged into the base of a cranial capsule, partly membranous, partly cartilagin ous. This skull, therefore, is not movable upon the spinal column. No vertebral segmentation can be Flo. 31. Upper (A) and side (I)) views, and vertical section (O, of the skull of Petromnion mariniu. o, notochord; 6, basis cranii ; c, inferior, and rf, lateral process of basis; e, auditory capsule;/, sub- y, Btylohyal process; A, olfactory capsule; i, ethmo-vomerine plate; i, palato-pterygold terspUCCS between the UCUrapOphyseS distance along the lower side of the spinal column, and a lateral, which is ramified into a skeleton supporting the branchial apparatus. A stylohyal process and a subocular arch with a palato-pterygoid portion may be distinguished. The roof of the cranial capsule is membranous in Myxine and in the larva; of Petromyzon, but more or less cartilagin ous in the adult Pdromyzon and in Udellostoma. A car tilaginous capsule on each side of the hinder part of the skull contains the auditory organ, whilst the olfactory capsule occupies the anterior upper part of the roof. A broad cartilaginous lamina, starting from the cranium and overlying part of the snout, has been determined as repre senting the ethmo-vomerine elements, whilst the oral organs are supported by large, very peculiar cartilages (labials), greatly differing in general configuration and arrangement in the various Cyclostomes. There are three in the sea- lamprey, of which the middle one is joined to the palate by an intermediate smaller one ; the foremost is ring-like, tooth-bearing, emitting on each side a styliform process. The lingual cartilage is large in all Cyclostomes. There is no trace of ribs or limbs. The Chondropterygians exhibit a most extraordinary diversity in the development of their vertebral column ; almost every degree of ossification, from a notochord without a trace of annular structure to a series of com pletely ossified vertebras, being found in this order. The sharks in which the notochord is persistent are the Ilolocephali (if they be reckoned as belonging to this order) and the genera Notidanus and Echinorhinus. In the other sharks the segmentation is complete, each vertebra having a deep conical excavation in front and behind, with a central canal through which the notochord is continued ; but the degree in which the primitive car tilage is replaced by concentric or radiating lamellae of bone varies greatly in the various genera, and according to the age of the individuals. In the rays all the vertebrae are completely ossified, and the anterior ones confluent into one continuous mass. In the majority of Chondropterygians the extremity of the vertebral column shows a decidedly heterocercal condition, and only a few, like Squatina and some rays, possess a diphycercal tail. The advance in the development of the skeleton in Chondropterygians beyond the primitive condition of the previous subclasses manifests itself further by the presence of neural and haemal elements, extending to the foremost part of the axial column ; but of these the haemal form a closed arch in the caudal region only, whilst on the trunk they appear simply as a lateral longitudinal ridge. The neural and hremal apophyses are either merely attached to the axis, as in Chondropterygians with persis tent notochord (the rays and some sharks) ; or their basal portions penetrate like wedges into the substance of the centrum, so that, in a transverse section, in consequence of the difference in their texture, they appear in flip form nf nn X (ficr* %<> Tlip in f the oculHrareh; portion of subocular arch ; /, m, n, accessory labial or rostral cartilages, with o, appendage; p, lingual V prtphr-T> arp not fillprl hvfil cartilage ; q. neural arches; r, branchial skeleton; ,, blind termination of the nasal duct between the vertet)1 lllecl b ) Q notochord and oesophagus. as j n O ther fishes, but by separate cartilages (laminae or cartilagines intercrurales), to which frequently a series of terminal pieces is superadded, which must be regarded as the first appearance of the interneural spines of the Teleostei and many Ganoids. Similar terminal pieces observed in the notochord, but neural arches are rcpre- Rented by a series of cartilages on each side of the spinal chord. In Petromyzon (fig. 31) the basis cranii emits two
prolongations on each aide, an inferior, extending for some
