riivi.ocicNY OK I'lii'; rKLi'.cvroDA. 



379 



<)(' llic Aviflllid.r ;m(l tlicir .•lilies. Niiciil;i is dim v.iiiiiii, ;iii .•iiidiHldf muscle e\is(iii'>' jil, 

 oillu'i' end ofliie e(|iiMl vidves. 'Tlie ^ills jii'i' in two pairs, (•(nii|»(ised uC siiuiili', disimiti'd 

 filiunents' (Milzidiiiri). Tlie mantle bm-dei's .-ii-e pinin, willrml (enliieles or n siplion, iiiid 

 Iree iiroiind llie niarj^ins. Tlie Tool is niarked hy ii loni^ilmrniid cleTl ; iic{r()i-din<>'lv it can 

 1k' llaKened into a eirenlar disc (^Mit/.idvuri ) and il lias h.'cn olis-rveil (aM\vlin<;' up tlie 

 sides ol" !i <;liiss of sea wati'i- ('rryou) iiidiciitiiifj^ coiisideraiile ( liiiL^in^" eapaeilA of llie 

 loot. .\11 these cliar.aelers eoiiieidiii*;' so closely with tlii' I'eatiires oC .anatomy and shell 

 struetiiiH" known in tiie prodissoconeli, (oj>'elher with the lael lli.at, Nueiila is Icnown in 

 very curly lossilileroiis lornialions coinhiiie to render it extremely proh.alile (hat, Niieida 

 or 11 Nueiiloid I'oriii is the type we are seekin*;' as the early aiieeslr.d radie.d repre- 

 si'iited by the completed pi'odissocoiich in the developineiil ol" .Vvieiil.a, I'erna, (Jstrca, 

 l*cclen, .\noiiiia .and their .allies. 



The fact that Nneiilais I'onnd in I lie liOWiT Slliiii.in lui nialioiis and still lives nplothe, 

 present day without sullieient chaiiiics in its shell striiel nre to mikellies(( extreinely 

 sep;irai)le I'ornis in lime t:,eiierieally separ.ahle, aryaies stron^'ly lor a very hi^li aiili(piity 

 Cor this <;-enus. it was so lirndy estahlished in its earliest known lorms Ih.al it musi .at, 

 that time have been an ancieiil <;-eiuis. It, may tlierelore well have been the primitivi', 

 aiicestr.al radical toward which the pi-odissocoiich of the <i,'enera under consideration 

 points as the ari'lietype, still indicated in the (le\ I'lopmcnl ol'a miih'rn individii.al. 



I li.ive, tlierelore, IVoiii the results ol'tlu'se stuilies been able to trace, to Nnciila or a 

 !Nneiiloid rorin as a probable type-ancestor, the prodissoconch which I have loiind to be 

 characteristic ol" di'velopin<>- Avicnla, I'erna, Ostri'a, I'ectcn, Auomia and tlu'ir allies. In 

 the next section (w i) the relationship cxistinj;" between liu'se sovcM'al j^'ciiera and their 

 allies is traced as lar as 1 am abli' to do so wit h my present ii iidcrsl a ndin^df the groups. 



XVr. ( Ji'Ni;.\i.()(;i('.vi. CoNNKOTioN ok tiik Aviciri.in.i.; and Tiiiau .\i.i,iks. 



It is shown in si'i-tion \v that llie prodissoconch, wirutli is chai'actcrislic ol' the Avic- 

 ulidii and their allies, as represented by typical <;\Miera, can b;' traced to Nneula or a 

 Nuculoid Conn as a probable ancestral radical of the whole j^'roup. In this section later 

 staj^cs ol" shell ;;a'owtli .are considered in their pliyloi;-eiietic bc;irinf>" and a t.ibul.ir view is 

 <»'iveii which represents the •^encaloj'ical councctioii oCtlie several i^eiicra as it is uiiih r- 

 stood Ci'om the results of lliesi' stiidii-s. I'^'ecpicnt reCerence is made to the prece(liii<;- 

 text. Cor a Cull consideration ol" points only brielly allniU'd to, and many •j;'ener<a not, pre- 

 viously studied are introduced. 



The eai'ly lu'pionic staj;c oC Avicula, li<;'s. '}2-3.'^, |). IJ.'JO, I'erii.a, IM. xwii, lii;-. I, and 

 Pccten, IM. xxviii, li<4". 9, is closely alike in many s])ecies, indicatini;' a common line oC 

 iviicestry Cor these jit'uci'a. 'VUc ])oiiits oC rcsiMiiblancc are .as follows: — Tin- body oCtlie 

 shell is sub-rhomboidal, obliipie, the liiiii;'e line str:ii<^lit ; the post(aaor winj;' is rounded 

 and is not produced iieyond the margin oC the body of the si id I. Anteriorly the straifj^ht 

 hinn'c extends beyond the limits of the prodissoconch, .and is leriniiiated \)\ a moderate 

 anrick' as in Avicul.a and Pccten, or the auricle m.ay descend directly I'rom the uiiibon.al 

 ari'a without aii\ anterior extension of the liiim-c line as in l*ern;i. In the several "■enera 



'Niiculii lias lii'iMi I'liMsiilcrril a |ii'lijiili\r in'olut y|i:il I'cl- 

 I'cypinl liy si'vinil iiivcsliHiilor,'. dii acciMiiii (if [hr sim- 



lillrlly nl'llir niUs, wlilciiiu'c panilli'lrd iiiUic early sla;;<'.s 

 111' many ni'iicni iil' IVli'cyixiils. 



