STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 777 



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 42, figs. 80 and Si, /./.), characteristic of Fishes generally, 

 and running continuously for the whole length of the vertebral 

 column. Each of the cartilaginous elements is, as will be after- 

 wards shewn, 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 corre- 

 sponding 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 carti- 

 laginous elements, and the osseous 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 

 Lcpidosteus. 



The posterior ends of the neural plates of the neural arches 

 are continued into the dorsal processes directed obliquely up- 

 wards and backwards, which have been somewhat 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 liga- 

 mentum longitudinale are placed. 



The vertebrae 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 

 recognizable. 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 immediately outside 

 the peritoneal membrane, along the bases of the intermuscular 

 septa. Their distal ends do not, however, remain close to the 

 B. 50 



