1046 



SCANDINAVIAN KISIIES. 



pii uj) into two parts, an iipjicr, wliich has medially 

 gi-(j\\'ii togetlier from the sides to form the framework 

 of tlic palatine roof and an analogue to the quadrate 

 bone (fig. 284, qu), and a lower (the Meckeliaii carti- 

 lage), which forias the framework of the lower jaw {de 

 + M). At the age of three months these teeth dis- 

 appear. Meanwliile the }(alato-(]uadrate cartilage has 

 differentiated into two tirndy coalescent, but distinguish- 

 able parts, the posterior {nipt) ans\\ering to the meta- 

 pterygoid of the Teleosts, the anterior corresponding 

 principally to tlieir pterygoid proper and quadrate (qn), 

 but also acquiring at its anterior margin special ossi- 

 fications, homologous with the palatines {pD and tlie ento- 



pa £::. 



Fis;. 285. Ossifications on tlic under surface of the skull in Acipen- 

 ser sturio. '/4 "^ the natural size. /),«/), parasphenoid ; pa, its ascend- 

 ing splienoid process; ro«(, vomer; sci\ vomerine scutes. 



pterygoids (pf). There further ap])e,'irs on each side of 

 the anterior margin of the ])alatine arch a freer bone, 

 the maxillary (lu.r), whicli extends from the anterior 

 tip of the palatine disk (pterygoid bones) to its lateral 

 extremity (the knob by means of which the quadrate 

 bone articulates with the mandible), leaving between 

 itself and the palatine disk a fissure, through which 

 the levators of the lower jaw find a passage. On the 

 outside of the above-mentioned juncture between the 



maxillarv and the quadi'ate lies a triangular ossifica- 

 tion {pop), nio.st jjointed aliove, which has been inter- 

 preted by Parkeh as representing a jtreoperculum; and 

 on the tiutside of the Meckelian cartilage (Jl/) is deve- 

 loped the dental part (r/e) of the lower jaw. 



lu the skidl itself there appear ossifications the 

 largest of which {psp) answers to tiie parasphenoid of 

 the Teleosts, and extends from tlie nasal region not 

 onlv under tiie skull, but also, divided into two lateral 

 ])lates (fig. 285), under that part of the spinal column 

 wliicli has not been completeh- divided into separate 

 vertebra', l)ut in tlie form of a cartilaginous Tuass, con- 

 tiiuious below, comp(jses a backward prolongation of 

 tlie ci'aiiial cartilage. This parasphenoid is covered 

 underneath, throughout the greater pai't of its length, 

 by the hard mucous membrane alone; lint between the 

 nasal and orliital regions (in tlie etlimoidal region) it 

 pierces the downward projection (fig. 284, B, the so- 

 called hasal angle, which may also be observed in the 

 Sharks) of the chondrocranium. Within tliis it meets 

 and wedges itself into the vomer {ram in figs. 284 

 and 28.5) in front of it, which advances under the 

 rostral cartilage (fig. 284, B), and is continued in its 

 turn by several vomerine scutes (fig. 285, scr), evi- 

 dently belonging to the skin. In tlie etlimoidal region 

 (the lower anterior part of each orbit) the lateral eth- 

 moids (fig. 284, eU) are developed; al)ove the foramen 

 of each optic nerve (in the arched roof of the orbit) 

 aj)pears an orbitosphenoid bone (fig. 284, ohsp), behind 

 this a smaller osseous disk, corresponding to the ali- 

 sphenoid, and behind the orbit, round the orifice for 

 the nervus tngemmus {tr), an osseous disk answering 

 to the petrosal. I'ut all these bones, according to Pah- 

 kek", are su[)erticial growths external to the cartilage 

 {parostoses), foreshadowing the cartilage-bones (ectosto- 

 ses and entostoses) present at the same points in the 

 Teleosts a-r:d the higher vertebrates. Similar foresha- 

 dowers apjjear in the form of osseous scutes (scale- 

 growths) in the skin on the toj) of the head and outside 

 the shoulder-girdle. At the middle of the ocei])ut, but 

 firndy united to the first dorsal scute, lies a trefoiled 

 (posteriorly broad, ^vith a narro\ver lobe projecting in 

 front) osseous plate, evidenth' answering to the supra- 

 occipital (here called the siipraoiTipifal snite, fig. 286, 

 ocs) of the Teleosts. Further forward lie two larger 

 scutes, usuallv the largest on the whole head, whose 



Pliilos. Trans. Roy. Soc. London, vol. 17.3 (1882), p. 175. 



