LAMPREYS. 
1173 
great morphological significance. In the structure of 
the skeleton of the Lampreys the difference between 
the internal (vertebral) and the external (extravertebral) 
parts is far more prominent than in the fishes we have 
hitherto considered, the latter parts being here endowed 
with a more preponderating importance. The purely 
vertebral parts appear in the Lampreys merely as the 
framework of the cranium and its nearest appendages 
and as very imperfectly developed neural arches (fig. 
343, arc ) on the sides of the myelon and the unaltered 
notochord {etch). In the vertical fins too cartilaginous 
rods are developed which are applied to the neural 
arches in such a manner that it has been proposed to 
interpret them as representing spinous processes both 
superior (neural) and inferior (hsemal). The skull itself 
is an almost straight continuation of the fibrous me- 
dullispinal sheath, which has been dilated, and which 
grows, on each side of the brain, to form a more or 
less complete roof for the cerebral cavity (tegmen cranii, 
t). Laterally and downwards the floor expands in three 
places on each side into flat processes. The foremost 
pair of these processes projects from the ethmoidal part, 
the posterior two pairs issue from the sphenoidal part. 
The former is evidently homologous with a palatine arch 
(pal), the latter answer to the upper parts in the sus- 
pensory apparatus of the mandibular and hyoid arches. 
But no mandibular arch proper is developed in these 
fishes. The tAvo posterior processes on each side are 
confluent at the base; but the anterior of them (ptg) 
groAvs outwards and forwards, in a loop below the eye, 
until it coalesces Avith the top of the foremost (palatine) 
process (pal). In this manner a flat ring is formed, 
surrounding an aperture (fenestra snborbitalis or fissura 
orbitalis inferior, fo), and composing the bottom of the 
etch 
Fig. 343. The cartilaginous skeleton of the anterior part of a Lampern ( Petromyzon Jluviatilis ), 3 / 2 nat. size. 
ann, annular cartilage of the oral disk; arc, rudimentary neural arches; aw, cartilaginous capsule of the auditory apparatus; hr , branchial basket; 
cell, ceratohyal cartilage; ctr, posterior cartilage of the roof of the mouth ( cornua trabecularum ) ; eph, epihynl cartilage; et, ethmoidal part 
of the cranial floor; etch, outer sheath of the chorda dorsalis; fo suborbital fenestra; la, anterior lateral cartilage of the roof of the mouth; 
lat, lateral labial cartilage; Ip, posterior lateral cartilage of the roof of the mouth; med, median cartilage of the roof of the mouth; n, ol- 
factory capsule, with the tubular orifice of the nasal duct; pal, palatine part of the roof of the mouth; phy, posterior hyoid cartilage; 
pro, pericardial cartilage; ptg, pterygoid part of the roof of the mouth; si, sublingual cartilage; sph, sphenoidal part of the cranial floor; 
spl, symplectic (? hyomandibular) cartilage; t, lower part of the cranial roof ( tegmen cranii). 
ann lat 
is invested in the developed Lamprey with a more or 
less complete cartilaginous capsule (£) originating from 
the skeletogenous layer round the anterior end of the 
notochord. The part first formed, the cranial floor, 
ansAvering to a sphenoidal region (sph) applies itself to 
and coalesces Avith the already present auditory capsules 
(ait). In a fonvard direction are developed the so-called 
cranial trabecula?, around the cerebral appendage (hypo- 
physis, posterior nasal duct) and the under-brain 
(infundibulum); and after their union in front into an 
ethmoidal part (et) they advance to the so-called cornua 
trabecularum (ctr), Avhich coalesce into a disk in front 
of the nasal capsule (n). UpAvards the cranial floor 
cranial orbit. To the top of the hindmost process is 
attached the hyoid arch proper. The upper part of 
the latter is an unbroken doAvmvard continuation of the 
said process, and represents an epihyal bone (eph)-, the 
loAver (ventral) part, Avhich is suspended at right angles to 
the loAver tip of the upper part, ansAvers to a ceratohyal 
bone (cell). A certain similarity to this structure of 
the cranial roof of the palate and pharynx Ave have 
indeed seen in the Chimeeras (fig. 294, p. 1065), Avhere 
a coalescence, suggestive of a development presumably 
very primitive, has taken place between the pterygo- 
palato- quadrate parts and the cranium. But the closest 
resemblance to this structure in the Lampreys meets us 
