A treatise on zoology . short of the edge of the caudalfin; it is almost, but not quite,perfectly straight, the end of thenotochord being just a little turnedupwards both in the larva and inthe adult (Koelliker [271], Budgett[67]), and enclosed with the nerve-cord in a cartilage sheath, as inLepidosteus and Amio. Althoughthe tail is outwardly symmetrical,there is reason to believe that it is not truly diphycercal, but hasbeen derived from a more heterocercal form. For, while the dorsalspines are separate from the radials in the epichordal lobe, in thehypochordal lobe the lepidotrichia rest on
A treatise on zoology . short of the edge of the caudalfin; it is almost, but not quite,perfectly straight, the end of thenotochord being just a little turnedupwards both in the larva and inthe adult (Koelliker [271], Budgett[67]), and enclosed with the nerve-cord in a cartilage sheath, as inLepidosteus and Amio. Althoughthe tail is outwardly symmetrical,there is reason to believe that it is not truly diphycercal, but hasbeen derived from a more heterocercal form. For, while the dorsalspines are separate from the radials in the epichordal lobe, in thehypochordal lobe the lepidotrichia rest on direct prolongations ofthe haemal arches as in typical heterocercal fins (Fig. 61, p. 101).Two points of interest are to be noticed in the anal fin : the proximalsegments of the radials still articulate for the most part Avith thehaemal spines, and some of them fuse together, as in the medianfins of some Osteolepidoti (Fig. 252); and the dermal rays are stillmuch more numerous than the radials (Fig. 265a). In the caudal. Fig. 20s. Reconstniction of the pectoral girdle andfin-skeleton of a larval Po}uptervs, enlarged.(After Budgett.) e, coracoid region ; /, fora-men ; ill, uiesopterygial cartilage plate; ?mt,metapterygium ; pr, propterygiuni; r, radial;se, .scapular region. POL YPTERIN I 297 and dorsal fins each lepidotrich is related to one radial. Theradials supporting the dorsal finlets are each formed of a singlepiece, which, as in the Holostei (p. 322), projects not at all beyondthe body into the base of the fin (Bridge [56]). There are four l)ranchial slits, and neither mandibular norhyoidean gill. The gill-lamellae project freely at their end into thegill-chamber; but there is more septum preserved than in thehigher Actinopterygii (p. 95 and Fig. 57). A double series ofcartilaginous rays supports the lamellae. A Icmarkable pinnateexternal gill is developedon the hyoid arch of thelarva (Fig. 272). The ven-tral air-bladder, with cellularwalls, and a synmietricalarterial blo
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