322 



ME. "W. K. PAEKER ON THE STRIJCTUEE AND 



and the anterior and posterior sphenoids, are now thoroughly hardened. The " spheno- 

 occipital synchondrosis " is of small extent, very thin, and a scarcely thicker tract of 

 cartilage remains between the posterior and anterior sphenoids ; yet these are each a 

 single bone at this stage. The perpendicular ethmoid and septum nasi are still 

 unossified ; but the inferior turbinals are almost completely, and the "lateral masses" 

 partially, converted into endosteal bone. The cribriform plate is soft, and so is 

 the snout (Plate XXXVI. fig. 1) ; but this latter is everywhere burrowed with vessels 

 prior to hardening. 



Beneath the skull we see a most compact building together of the palatal, pterygoid, 

 external pterygoid, and tympanic bony pieces ; the thick, clubbed " hamular process " of 

 the internal pterygoid is fixed as an undersetter to the solid nut-like tympanic, and dints 

 it as an inturned horn dints the frontal in certain varieties of the Cow ; this, however, 

 is only a temporary state of things, and is quite recovered from in the lengthening head 

 and face. 



No " interparietals " have been found, adding two bones to the growing superoccipital, 

 as seen in Man ; this part, the superoccipital, is now a nearly vertical wall, the parietals 

 finishing the roof above. The lower jaw is well ossified, and is now entirely free from 

 the arrested primordial bar, Meckel's cartilage. The three periotic centres have 

 completely ossified the auditory mass, " petrosal " and " mastoid " (Plate XXXVI. fig. 2) ; 

 and a small bilobate ossicle has appeared in the attached (confluent) head of the stylo- 

 hyal (stJi.). Further down another centre has appeared in the middle of the long rib- 

 like bar, taking up nearly the middle third. The upper piece of bone (formed from 

 two nuclei in the Lamb, and apparently also in the Pig) is called by Professor Flo wee* 

 the " tympano-hyal," a term it may be well to retain. The rudimentary stylohyal 

 of Man is ossified from the upper centre ; for " a centre of ossification appears in the 

 styloid cartilage, and extending upwards and downwards, gives rise to the pyramid and 

 styloid process" (Huxley, 'Elem.' p. IGO). Hence it will be seen that the tympano- 

 hyal and the n^2)er styloid bone are identical ; both these bones are largely developed 

 in the Osseous Fish, the so-called " epi- and " ceratohyals ;" they occupy the great fiat 

 "cornu," at the base of which the short ceratohyal proper, with its two bony centres, 

 is articulated. The unciform ceratohyal (" cornu minor ") of the Pig is strongly attached 

 by fibrous tissue to the transverse basal piece (Plate XXXVI. fig. 2, c.k), now coalesced 

 with its own rudimentary arch, the "cornua majora" of Man; these pieces are ossified 

 proximally (fig. 2, th.h.), and these centres correspond with the first pair of hypo- 

 branchials of the Osseous Fish, the median part answering to the first basibranchial. 

 The " stylo-mastoid foramen " (fig. 2, s.m.f., 7") is seen transmitting the portio dura 

 nerve ; and this sends upwards and forwards the " chorda tympani " (7"'), to which is 

 attached the smallest of the three " ossa bulla; " (o.b'.) ; the middle-sized piece is seen 

 in fnmt of the stylohyal (o.b".). The rest of the drum-walls being removed and 

 the squamosal, the outer face of the periotic mass shows the three semicircular canals 

 * In his vahiahlo little work ' On the Osteology of the Mammalia,' 1870, p. 17:5. 



