The evidence afforded by the mode of formation of the skeletal tissues of vertebrates down to the Elasmobranchs indicates that the primitive cranial skeleton arose from two paired basal cartilages, the parachordals and trabeculæ, to which were attached respectively cartilaginous cases enclosing the organs of hearing and smell. In addition, the branchial portion of the cranial region was provided with cartilaginous bars arranged serially for the support of the branchiæ, with the exception of the foremost, the mandibular bar, which formed supporting tissues for the mouth--the upper and lower jaws. Just as in past times the spinal nerves and the segments they supplied were supposed to represent the type on which the original vertebrate was built, so also the spinal vertebræ afforded the type of the segmented skeleton, and the anatomists of those days strove hard to resolve the cranio-facial skeleton into a series of modified vertebræ. Owing especially to the labours of Huxley, who showed that the segmentation in the head-region was essentially a segmentation due to the presence of branchial bars, this conception was finally laid to rest and nowadays it is admitted to be hopeless to resolve the cranium into vertebral segments. Still, however, the vertebrate is a segmented animal and its segmented nature is visible in the cranial region, so far as the skeletal tissues are concerned, in the shape of the series of branchial and visceral bars. To this segmentation the name of 'branchiomeric' has been given, while that due to the presence of vertebræ is called 'mesomeric.' As we have seen, the internal bony skeleton of the vertebrate commences as a cartilaginous and membranous skeleton. For this reason the preservation of such skeletons is impossible in the fossil form, unless the cartilage has become impregnated with lime salts, so that there is but little hope of ever obtaining traces of such {125}structures in the fossils of the Silurian age either among the vertebrate or invertebrate remains. Fortunately for this investigation there are still living on the earth two representatives of that age; on the invertebrate side Limulus, and on the vertebrate side Ammocoetes. The Elasmobranchs represent the most primitive of the gnathostomatous vertebrates. Below them come the Agnatha, known as the cyclostomatous fishes or Marsipobranchii, the lampreys (Petromyzon) and the hag-fishes (Myxine).