STRUCTURE AND DEVELOPMENT OE LEPIDOSTEUS. 
387 
fibre-cartilage), which rest upon the roof of the neural canal. Each element is situated 
intervertebrally , its anterior end being wedged in between the two dorsal processes of 
the neural arch of the vertebra in front, and its posterior end extending for some 
distance over the vertebra behind. The successive elements are connected by fibrous 
tissue, and are continuous dorsally with a fibrous band, known as the ligamentum 
longitudinale superius (Plate 29, figs. 80 and 81, l.l), characteristic of Fishes generally, 
and running continuously for the whole length of the vertebral column. Each of the 
cartilaginous elements is, as will be afterwards shown, developed as two independent 
pieces of cartilage, and might be compared with the dorsal element which usually 
forms the keystone of the neural arch in Elasmobranchs, Were not the latter vertebral 
instead of intervertebral in position. More or less similar elements are described by 
Gotte in the neural arches of many Teleostei, which also, however, appear to be 
vertebrally placed, and he has compared them and the corresponding elements in the 
Sturgeon with the Elasmobranch cartilages forming the keystone of the neural arch. 
Gotte does not, however, appear to have distinguished between the cartilaginous 
elements, and the osseus elements forming the roof of the spinal canal, which are true 
membrane bones; it is probable that the two are not so clearly separated in other 
types as in Lepidosteus. 
The posterior ends of the neural plates of the neural arches are continued into the 
dorsal processes directed obliquely upwards and backwards, which have been some¬ 
what unfortunately described by Stannius as rib-like projections of the neural arch. 
The dorsal processes of the two sides do not meet, but between them is placed a 
median free spinous element, also directed obliquely upwards and backwards, which 
forms a kind of roof for the groove in which the cartilaginous elements and the 
ligamentum longitudinale are placed. 
The vertebrse are wholly formed of a very cellular osseous tissue, in which a 
distinction between the bases of the neural and haemal processes and the remainder 
of the vertebra is not recognisable. The bodies of the vertebrae are, moreover, directly 
continuous with the neural and haemal arches. 
The ribs in the region of the trunk are articulated to the ends of the long haemal 
processes. They envelop the body cavity, their proximal parts being placed imme¬ 
diately outside the peritoneal membrane, along the bases of the intermuscular septa. 
Their distal ends do not, however, remain close to the peritoneal membrane, but 
pass outivards along the intermuscular septa till their free ends come into very close 
proximity with the shin. This peculiarity, which holds good in the adult for all the 
free ribs, is shown in one of the anterior ribs of an advanced larva in Plate 28, fig. 72 ( rb .). 
We are not aware that this has been previously noticed, but it appears to us to be a 
point not without interest in all questions which concern the homology of rib-like 
structures occupying different positions in relation to the muscles. Its bearings are 
fully dealt with in the section of this paper devoted to the consideration of the 
homologies of the ribs in Fishes. 
