386 
MESSRS. E. M. BALFOUR AND W. N. PARKER ON THE 
Muscular System. 
There is a peculiarity in the muscular system, of Lepidosteus, which so far as we 
know has not been previously noticed. It is that the lateral muscles of each side are 
not divided, either in the region of the trunk or of the tail, into a dorso-lateral and 
ventro-lateral division. 
This peculiarity is equally characteristic of the older larvae as of the adult, and is 
shown in Plate 28, figs. 67, 72, and 73, and Plate 29, figs. 74-76. In the Cyclostomata 
the lateral muscles are not divided into dorsal and ventral sections ; but except in 
this group such a division has been hitherto considered as invariable amongst Fishes. 
This character must, without doubt, be held to be the indication of a very primitive 
arrangement of the muscular system. In the embryos of all Fishes with the usual 
type of the lateral muscles, the undivided condition of the muscles precedes the 
divided condition; and in primitive forms such as the Cyclostomata and Amphioxus 
the embryonic condition is retained, as it is in Lepidosteus . 
Skeleton. 
Part I.— Vertebral column and ribs of the adult. 
A typical vertebra from the trunk of Lepidosteus has the following characters 
(Plate 29, figs. 80 and 81). 
The centrum is slightly narrrower in the middle than at its two extremities. It 
articulates with adjacent vertebrae by a convex face in front and a concave face 
behind, being thus, according to Owen’s nomenclature, opisthocoelous. It presents 
on its under surface a well-marked longitudinal ridge, which in many vertebrae is only 
united at its two extremities with the main body of the vertebra. 
From the lateral borders of the centrum there project, at a point slightly nearer 
the front than the hind end, a pair of prominent haemal processes ( h.a .), to the ends 
of which are articulated the ribs. These processes have a nearly horizontal direction 
in the greater part of the trunk, though bent downwards in the tail. 
The neural arches (n.a.) have a somewhat complicated form. They are mainly composed 
of two vertical plates, the breadth of the basal parts of which is nearly as great as the 
length of the vertebrae, so that comparatively narrow spaces are left between the 
neural arches of successive vertebrae for the passage of the spinal nerves. Some little 
way from its dorsal extremity each neural arch sends a horizontal process inwards, 
which meets its fellow and so forms a roof for the spinal canal. These processes 
appear to be confined to the posterior parts of the vertebrae, so that at the front ends 
of the vertebrae, and in the spaces between them, the neural canal is without an 
osseous roof. Above the level of this osseous roof there is a narrow passage, bounded 
laterally by the dorsal extremities of the neural plates. This passage is mainly filled 
up by a series of cartilaginous elements (Plate 29, figs. 80 and 81, i.c .) (probably 
