■50 iUILLKTIN lil, KMTKU .STATKS NATIONAL MIJSKUM 



(xTiitoli.val iscitlier lice ;il its posterior I'-xticiiiilv, iis in IIh^ Proteidii, 

 or in tlic more tvpiciil lonns it Joins tlic (iniulralc ('artiIaf,'o Ity «lirect 

 ai-tii'iilation or by li-ainiiit. 



TIicIi.vomI a]>|i,tiaiiis lias a very uniform cliaractiT in tlic various 

 fiiNiilics. ('.\ci'ptiM,u- in tlic CrvptolnMiu'liiida', Aniphiurnida', and Co'cil- 

 iidif, wlifrt' it is (piite ditlVrtMit. In tiie typical forms it is constructed 

 as follows: lis si:si)(iisor is, as usual, tiie eeratoliyal, wliicli is undi- 

 vided. This is eoiiiiected witii a median hasal piecte, the i'lvat Uasi- 

 branchial, i»y a sej-ineiit, i^enerally slendei', the hypobyal. Soinelinies 

 tins piece docs not touch the cciatoliyal. Imoui the posterior lateral 

 border of the basibraiu-hial there orii^'inatcs on each side a rod, the lirst, 

 cerafobranclnal, which is continued after a Joint, as the first eidhran- 

 chial. From a i)roccss which orij^inates between the bases of these, on 

 flic siijicrior side of the Iiasibranchial, there oii,!4inates on each si<le of 

 its postciior extremity another rod, shorter than (he first, the second 

 ceiatoiiranchial. In some {(cncra (Ilyuobiida-) tlieie is attaciu'd to the 

 (b'stal cxticmity of this lod a second one, continuous witli it, the secoiu! 

 eiiibraiicliial. The second basibranchial is separated from tl: > iirst, 

 and is situated far posterior to it, not far in advance of the corac(tid 

 bones. It is better ossilied than iiutst of the rest of the liyoidx, and was 

 called the thyroid bone by Ciivicr. Thatitis the distal extremity (d' 

 the second basibranchial is shown by the fact that it is continuous with 

 the first by a median rod in the larva-. (Plates l.l, l',">, I'O, ;}(», ,'{7.) 



The important differences disi)layed by the different forms of the 

 liyoid ill the Urodcla may be tabulated as follows. 



I. Mi'<liiiii 1111(1 l;itfi;il clciiicnls ilistiiic't. 



((. Two ('('liltdlllMllcllinl puii's. 



CVi'iitDliyals mi(li\ idcil ; ciiilirjiiicliiiils, lir.sl mid laroly sfcond iircsciit. I'siudu 



Kuitriu lilv. 

 CVr;it<>liy:ils(livi(l(<l; t'liiliriimliials, •,>,:!, and l [ncHi'iit. TrcmiitotUni Umii. 



aa. Oiii' i)iiir oftcratoluaiicliials. 

 Coratoliyalsiiiidividrd; ciiibiaiRdiials, ',>, ;), and 4 prctsi'iit. tmiihiiinwidm 

 C'opi'. 

 II. Lateral f' 'MciiIs conliniKnis across middle line. 



Xom-UiaiMlnmnls (/w?a OppH.' 



The Trematodcr.i, Ampliiiimonlra, ami Ai)odii contain one family 

 each, viz: the Cryiilobriiiichida', .VinpliijiMiibe, and ('(elciiidic. The 

 value of the differences scpnratiii- them is diininislied by the fact that 

 in the larva' of the I'.scndosauria the 1, 1', .!, and J epi'biMiichials arc 

 present, and the eeratoliyal is lrc(pieiitly divided (c. 7, VJunubotits tauhro- 

 sits, Plate LT), fi- 1). 



The subordinate diiniviices presented by {\w families and genera 

 are indicated under their rcs]icctive heads. 



The Trodela. excepting the Apo.la. have limbs. The toos number 

 from two in both IW1 to five in flic posterior foot, and four in the anteri.ir 

 foot.^ No r,ro.h-lc a-rces with theextiiKtt .oe„„s Tlryops (Cope), from the 

 *0(.p''l, 1,^11, n„i ./y,,W,.,of L.mia'as(i„ lidiesj; r.vahpkitiiaD^liL, Irtlti. 



