324 Royal Society : 



the osseous fishes present the same complications of structure as 

 the Plagiostomes, if it is true that the bodies of their vertebrae are 

 developed from the proper sheath of the chorda. So, for instance, 

 there exists a beautiful elastic internal layer outside of the remnants 

 of the gelatinous chorda in the genus Orthagoriscus. 



B. Form of the chorda proper. 



1st. The chorda retains in some instances its original cylindrical 

 form ; and this is the case when the vertebral column shows no indi- 

 cation of vertebral bodies (Cyclostomes, Acipenser, Chimcera, Lepi- 

 dosiren, Tilurus, Hyoprorus* anterior vertebra), as well as where 

 vertebral divisions exist (Leptocephalus, Helmichthys, Hyoprorus 

 last vertebra). 



2nd. In other cases the chorda is contracted in the middle region 

 of each vertebral body, which seldom happens where there is no 

 trace of ossification (Hexanchus), but is very generally the case in 

 ossified vertebrae (Squali, osseous fishes, perennibranchiate amphibia, 

 Coecilice) . 



3rd. Lastly, the chorda may be separated into as many parts as 

 there are interstices between the vertebrae, which remaining parts in 

 some cases are totally absorbed (Raia and most of the higher 

 animals) . 



C. Anterior end of the chorda. 



1 st. In many full-grown fishes the chorda dorsalis reaches with 

 its anterior attenuated end to the base of the cranium, and its cranial 

 part is in some cases enveloped in its whole length by the cranial 

 cartilage. This fact has been long known with regard to the Aci- 

 penseridse, Cyclostomi, and Sirenoidei ; but the same thing occurs 

 amongst the Squali, and has been observed by Stannius in Prionodon t 

 and by me in Heptanchus, Centrophorus, Acanthias y and Squatina. 

 In these last fishes the chorda reaches as far as the region of the 

 hypophysis, and is bent upwards at its termination, so that the end 

 itself lies underneath the interior perichondrium of the cranium, or 

 at least very near the surface of the cartilage. In other cases only 

 the hinder part of the chorda is enclosed by the cranial cartilage, 

 whilst the anterior half lies in a groove at the under part of it, as in 

 Leptocephalus and Helmichthys. In one case (Tilurus) the whole 

 cranial part of the chorda is free, and situated underneath the base 

 of the cranium, between its cartilage and the perichondrium f. 



2nd. In some genera of Squali and most of the osseous fishes, the 

 cranial part of the chorda is reduced to the anterior half of the first 

 ligamentum inter vertebrale. 



3rd. In the genus Chimcera, the chorda ends in the foremost part 

 of the vertebral column. In this case the connexion between the 



* Two genera belonging to the Leptocephalidse, described by me (see Kaup, 

 Apodal Fishes of the British Museum. London, 1856). 



f In all these fishes there exists rather a strong connexion between the vertebral 

 column and the cranium ; in Squalina, besides this there are two lateral articulations 

 between the cartilaginous arches of the first vertebra and the lateral parts of the 

 cranial cartilage. 



