OCR Text |
Show 270 MR. ST. GEORGE MIVART ON THE [Apr. 28, oblique one which traverses (in a downward and backward direction) the outside of each hypapophysial arch. This condition is perhaps best exemplified in the caudal vertebrae of Chioglossa, where the transverse process is in just such a condition as would be one of those of the midtrunk of Siren, if its tubercular and capitular parts were so reduced as to be mere prominent ridges on the side of the centrum instead of strongly projecting, more or less horizontal plates. Thus in Siren we have the tubercular process extending downwards and backwards from the interzygapophysial ridge and ending in a backwardly projecting process, which is also the termination of the capitular process, the free edges of the plates forming an angle open forwards. Now in Chioglossa we have a ridge (the representative of the tubercular part of the transverse process) running downwards and backwards from the interzygapophysial ridge, and terminating in a backwardly projecting process, which process is also the termination of another ridge which runs forwards and downwards from it, and is the representative of the capitular part of the transverse process. This latter ridge, as it descends, unites with the ridge traversing obliquely the outside of the hypapophysial arch, and terminating behind and below in one of the posteriorly projecting processes of that arch hereafter described. Fig. 10. Lateral view of anterior caudal vertebrae of Menobranchus (No. 582 A in College of Surgeons' Museum). t. Tubercular process, c. Capitidar process. Hy. Hypapophysis. Sometimes both the capitular and tubercular parts of the transverse process are to be seen distinctly projecting out (though of small size) one above the other (fig. 10, c. t), from the side of a caudal vertebra, as e. g. sometimes, at least, in the fourth caudal vertebra of Menobranchus. Occasionally a certain osseous connexion exists between the caudal transverse process and the hypapophysial arch, or caudal hypapophyses. This is the case sometimes in the first one or two vertebras of Siren which possess hypapophyses. In Menobranchus also a bony connexion distinctly exists, in the third and fourth caudal vertebrae, between the root of the hypapophysial arch on each side and the under surface of the capitular process above it (fig. 15). This shows a certain degree of imperfection in the separation of the hypaxial part of the skeleton from the paraxial part. |