[J. Müller, Ueber den Bau und die Lebenserscheinungen des _Branchiostoma lubricum_, in Abhandl. Ak. Wiss. Berlin, 1844.] The skeleton of the _Cyclostomata_ (or Marsipobranchii) (Lampreys and Sea-hags) shows a considerable advance of development. It consists of a notochord, the anterior pointed end of which is wedged into the base of a cranial capsule, partly membranous partly cartilaginous. This skull, therefore, is not movable upon the spinal column. No vertebral segmentation can be observed in the notochord, but neural arches are represented by a series of cartilages on each side of the spinal chord. In _Petromyzon_ (Fig. 30) the basis cranii emits two prolongations on each side: an inferior, extending for some distance along the lower side of the spinal column, and a lateral, which is ramified into a skeleton supporting the branchial apparatus. A stylohyal process and a subocular arch with a palato-pterygoid portion may be distinguished. The roof of the cranial capsule is membranous in _Myxine_ and in the larvæ of _Petromyzon_, but more or less cartilaginous in the adult _Petromyzon_ and in _Bdellostoma_. A cartilaginous capsule on each side of the hinder part of the skull contains the auditory organ, whilst the olfactory capsule occupies the anterior upper part of the roof. A broad cartilaginous lamina, starting from the cranium and overlying part of the snout, has been determined as representing the ethmo-vomerine elements, whilst the oral organs are supported by large, very peculiar cartilages (_labials_), greatly differing in general configuration and arrangement in the various Cyclostomes. There are three in the Sea-lamprey, of which the middle one is joined to the palate by an intermediate smaller one; the foremost is ring-like, tooth-bearing, emitting on each side a styliform process. The lingual cartilage is large in all Cyclostomes. There is no trace of ribs or limbs. [J. Müller, Vergleichende Anatomie der Myxinoiden. Erster Theil. Osteologie und Myologie, in Abhandl. Ak. Wiss. Berlin, 1835.] [Illustration: Fig. 30.--Upper (A) and side (B) views, and vertical section (C) of the skull of _Petromyzon marinus_. _a_, Notochord; _b_, Basis cranii; _c_, Inferior, and _d_, Lateral process of basis; _e_, Auditory capsule; _f_, Subocular arch; _g_, Stylohyal process; _h_, Olfactory capsule; _i_, Ethmo-vomerine plate; _k_, Palato-pterygoid portion of subocular arch; _l-n_, Accessory labial or rostral cartilages; with _o_, appendage; _p_, lingual cartilage; _q_, neural arches; _r_, Branchial skeleton; _s_, Blind termination of the nasal duct between the notochord and œsophagus.] [Illustration: Fig. 31.--Heterocercal Tail of Centrina salviani. _a_, Vertebræ; _b_, Neurapophyses; _c_, Hæmapophyses.]