developed, is only from half a line to three quarters of a line, and is often less. Commenc- 

 ing at the beak and extending to the front of the ventral valve i- a well-detined mesial .-inu-. 

 which is usually shallow and rounded, and is bounded laterally by plications -tronjer than 

 the rest. 



Dorsal valve moderately convex, not so convex as the ventral valve, it- b.-ak incurved, 

 but not in contact with that of the ventral valve, its area extremely narrow and linear. < '.>r- 

 re-iioiidin" with the sinus of the ventral valve, there is a prominent mesial fold extendin- 



I * * 



from the beak to the front margin. 



Surface marke.l by -ub an-ular radiatin- ribs or plications usually from ei-jht to twelve 

 on each side of mesial fold and -inus. the outermost not reaching the beak, and the cardinal 

 angles being free from them. The plications are cro.--.-d by numerous imbricating /!_' 

 concentric striae, which become rery much crowded together a- the mar-in of the shell i- 

 approached, and which, close to the mar-in, are often interrupted b\ -tron-er lino of growth. 



The dim. n-i> ns and form oi' ^ "/" ar -ivdy variable hO much .-.> 



th.-it the extreme terms of the series have all the appearance of being distinct species. The 

 loliowin- jives the form and mea-un-ments of the leading varietie- of thi- D in 



the Hamilton Formation of Ontario, where it i.- a wonderfully abundant species, and i- found 

 in a state of beautiful preservation : 



'i. A semi circular, and very jibbous form, with the cardinal anjle- acute, but hardly 

 produced, width of hinge-line one inch; length eijht line.-; hei-ht -even Hi. 



A moderately convex form, with the cardinal aiules niodei-ate|\ but not exoessh 

 produced; the -.en.-ral lijure of the shell becoming trigonal. Width at hin-e line one inch and 



n line- : length, eleven lines j thickn- nlinea This may be regarded as the normal 



form of the specie-. 



--. A form upon the whole resembling the preceding in outline, but with -trai-hter -id.--. 

 that the -hell is of a regular triangular shape, theapexofthe triangle being truncated at the 



me-ial .-inu.- and fold. The shell is also much thinner nwinj to the -mailer comcxity of the 

 valves, and -] .-,-ially of the dor.-al valve, which i- j.-ntlv arched near the nmbo. but i- m 

 or le-s abruptly reflected all round the mai-jin at about tw -third- of its lenjth from tin- beak. 

 The cardinal EU re also more extended, inci-.-a-in- tin- disproportion between the width 



andlen-th. Width. -it hinge-line, twenty-two lines j l.-njili nine line-, height, only four Ii 



This foim occurs not uncommonly in the railway cuttinj at Widdi-r. ami can !.- traced by 

 in-, n-ihle -ra'lation- into the piveedin-j. ju.-t as that -railuate- into the form lir-t dc-cri: 



<l. Lastly, a form in which the cardinal angles of the shell are immensely produced, the 

 total width at the hin-e line thus cominj to be three, tour. live, or even ,-ix tin 

 the l.-n-th at the beak-. Th.-i. [go, from fifteen to twenty plication- on either side of th. 



-ial fold and sinus. \Vi.lth at hinjc line in a -mall specimen twciitv two line- ; lenjth. 

 n line- ; heijit. three lim-- and a .piart.-r. \Viilth at hin-e-Hne in a I'u'l :neii 



three inches: l.-n-jtli. six line-: h.-i-ht apparent Iv between four and live lin 



/, ' Very abundant and be-nitifully pre-crvcd in the decompi 



shal. -of the Hamilton ji"Up, on the line of the i irand 'I'nink llailvvay. near Widder Station. 



V\he|-e onlv the lil'-t three of the Vatielie- here described arc knoWII to occur. \l->. e mi: 



in the. .- us or calcareo-arenaceous beds of the same f- rm-ition at I'.anlett - Mi 



Aikona, in the Town-hip of l',o- m.| i-t. vvli.-re t h .-xt p-np-ly mucroirit.- variety i- thccb: 

 or only, on.- i -cur. Also, in tin- ( 'ornili-roii- Li IIP-- tone of I'm t t '..'.borne an l;i : 



:7 Si-iKin i: v \ A itn "- v i Hall). 



,^ / i Hall.. T.nth K'.pori on the State Cabinet, p. !"" 



s/ < i;illi' J Vol. VI., p. 255, 1 

 64. 



Spi II Vol. H \ \ \ I i 



Thi ith w hich it 



lorm and i' I The chief point by which - - di-i in .-ui-h.-.l i- lh. 



eomparaii\. of tip- \ , ntra! ai . a. and the prominent 



Tim- ii; the vditra Italf a line in height, whilst ii. 



it m v> hi\.- a hei-ht of a- much a- two i a half. 'I 



6 



