37S ROHKKr rUACY JACKSON ON IHK 



l>e»\ Those two lines of ii\quirv Hukeil with the proper paljvoutoK>g'ieal soi^uenee ane 

 fouiul to p.Mnt iliivellv to a tbnu whieh tills the neoessarv ivnuiivmeuts ofsueh an amvs- 

 toi\ Data tor the iuipiiry. The shtlL In Avicula, IVrna. Osiiva, Peeten. Anoniia 

 and allies a pj\>iUssi^ev>neh exists in whieh the struetur** is honiogvneons anil laniinar, 

 stU>-naeivous, wot prismatie, though siiects?t\ecl bv a dissvXH^neh having- well-detined pris- 

 matic struetutv. The iuulx>5i of the pi\Hlissixi'>nehs aiv liirtH'teil p^^steriorly, aiul the 

 hingv line is eurvevl. The onlv genus in whieh we know the pi-odissoiiiiueh before auv dis- 

 so<H>neh gn^wth has taken plaet\ so to enable us to determine feature's of the hinge line, 

 at this stage is Ostrea. In (}s!titit etluiis, tig. 2(», p. 312, the jirvxlissoiinieh has teeth 

 at right angk\!i to the hingv line, and these ttvth closely i-esemble the teeth of Xncula, 

 Avhen a -;^\-'' ;'f that gvnns is viewed from the nmb^" •■ -J:' ^•', as in the tigure cited. In 

 O. fir> , hi^wever, no ttn^th were oWervtnl. i y as illustrateil bv the va- 



rious g"enera yields points of great synthetic valne. The procliss;xx">ueh has two avl- 

 dnctor mnscle* as proven in Ostrea,I*i. xxiv, tigs. 1-2, and inferreil in the other genera 

 fivm the fv>rm of the pi\HVis*toconc-h valves and the kinship to Ostrea (see studies of 

 Perua, Avicula, Peoten and Anomia). In the prodissoct>nch stage of Ostrea the gills 

 are simple unjvaired filaments and they are evidently simple filaments at first in other 

 arenera as indieatevl in ottr studies of Anomia and Peeten.^ The mantle border of the 

 prvHlissoeoneh is free arv^und its entire margins except at the hinge line in Ostrea, and 

 beai"« no tentacles v^r siphons- A similar ci>ndition of freedom exists in the yonng\L»st 

 stag^'-s we know of related gvaera, and the development of tentacles and eyes described 

 in Peeten indicates that at a very e{»rly period the borvler was without them in that ge- 

 nxts, A fi>ot evidently exists in the tyjwcal prvKlissvxxynch, for though wanting in (.>>- 

 trea,* it is pivsent and highly develojHxl at the cU>se of the pi\Hliss<.HX>nch jvriixl in the 

 vonng of Jiiany rv>lated gvnera (Avicula, Perna, Peeten, Anomia, etc.>, as 1 have shown. 

 The fv,H>t where known is markcil by a pi\x\lmal byssal cleft and a distal cleft which lat- 

 ter IS flattened out when crawling, giving the toot a disc-like, elinging power (see Pec- 

 ten a\id Ant>mia, sections ix and xii>. 



WtT ^'^ -■ ■ ' .-■••.^ 7;,,.., . .t ,v .^. ..^^ ,^,. ^-,, „.,, .J. ,-^ ,,.^ ^..„./., ^^|. 



ywjx'c>>.- . ^ .- , . . / ^ L >-e characters ar. , to be i. .1 iu 



a retnarkable degive in the ancient genns Xncula which, though living tonlav, extends 

 back to very -e^wly fossilifervnis formations, fitly spe«eis (Tryon) being known from the 

 pa' Nucttla is one of the few g<»uera known which hss the umbos of the shell 



diivv .V -> > -T rT. . .< ,<-,. r ..• .M T .>i:*the body.* Tae shell of Xucula is oval, nacreous, 

 not pr - ~ ^^e line with teeth at right angles to its axis. A tri- 



angttlar, central cartilage pit is a characteristic feature of the Xactilid», and though it 

 is not definitely known to exist in the prodisstxvnch it may exist at that stage in some 

 senei^a, and is a marked feature of the succeetliug dissoconch in all the ascendent series 



^ " - - - Mf tl' ' ' ^ ' - VoaiCtibw IVa&x imi Tri- 



, ..•>t» i -*««? ftumly. 



*'r*io a^seoce^Qf Uw &JO« t«t V>!*t<.'a 8* ftjliy (fistttessi^l itt 



■'L_. _.,-uswi oiiSsivie tile Xuculula?, iuwtuctli tile uju- '. -, ~ , ■ ^-Cc. 



