87 



Ath>/ri.i '/</</ (Billinirs), C ' N--\v S--ri.--, Vol. \ . p. i'7L Pigs 29 



.)/ (Hall), / . v York, V..1. IV.. Plate XLVII1 . !';_>. i 2 



" Nearly smooth, ovate or rab-rhomboidal ; irreate-t width, about tHe middle ; a .-hurt 



linguiform projection in the middle of the front mar-in ;b .th valve.- convex ; ventral valve the 

 larger, with its beak .-trungly iucurveJ. Length from one to two im-he-: width eijual \ 

 little less, or a little LTeater, than the- length. The Ordinary si/.-- i- about one inch ami a halt in 

 length. 



" The ventral valve is -trun-ly and -moothly convex, the outline evenly an-hed from 1 

 to front, more abruptly curved above than below, the uml><> prominent, the b--ak rather -n. 

 ir-atly rounded at the -id---, and el-i-ely incurved. The lin-uiform projection in the mi- 

 of the front mar-in i- often a -imple exten-i.m ol' a portion of the -h--l.. without a Him-, hut 

 occasionally there is either a -hurt, shallow d- -pre--ion. or a narrow rounded me-ial fold, whieh 

 seldom, however, extends towards the h.-ak more than three or four line-. The upper ha> 

 the dorsal valve is sometime- - -\ --nly convex, but in general an indi-tinet. more or . 

 rounded caririation. can be traced from the umbo aloir_ r the middle t-i th- JV'-nt. where it 

 become- abruptly elevated into a short, promim-nt. rounded f,.ld, which extend- into the lin-ui- 

 form projection. On each side of the median line, this valve i- _.-nily convex, and often exhi 

 bits a rather flat slope to the lateral nnrjin-. The beak i- >tr-.n-l\ incurved, and ap; rs ( 

 b.- do-ply buried in the cavity beneath the umbo of the ventral va; . 



11 if a line be drawn aen>s- the -hell at mid-l--n-_'th. and an-itlier at one fourth the lcn-th 

 tVi-m the front, the greatest width will be found to. range between tin- two. M.mv -(' ti, - 

 niens are obtusely angular at the -id---, and in .-uch the mar-in- above and b--!ow th. 

 UP- -oiuewhat -trai-ht, the upper two >id--- converuin- to the beak, ami the lower :wo t> the 

 linguiform extension in front, -ivin- to the -hell a rh-nnb-iidal in.-tead of an o\at-- -mtlin.-. 



At lir-t -i-ht. the surface appears to !,. -niouth. with a few concentric -.juam-.-e lin,--, 

 of irrowth. On closer examination, mim-Ton- indistinct, radiating lines may be seen. < if tl 



thei-- are from two to four in the width of om- line, and they -ometim-- appear t. li.- ben-- ill, 

 tin- -urface of the shell. In very peri'.-ct -p---im'-n-. the -iirfae,. exhibit- fine c-im-.-ntric -t: 

 from ten to fifteen in th.- width of one line, and the-e are ino-t di-tincl to\var-l- the front 



margin 



" Beneath the 1- ik of the -l.-tacln-d ventral valve-, there i- a wide, trian_ r u'-ir f.-ramen. not 

 vi-il. e when the valve- are united. The in-i-le of the beak i- entirely hollow-.d out int- 



pit or channel, which opens directly into the ca\ity of the -hell. The impi-e. ion- of the 

 di\ ,i] 'catur lun- u],y part of a sub-triangular space, the upper angle of which is sitnal 



jn- where the excavation beneath the tu-ak open- mit into the vi.-ceral cavitv. Thel-,\\,r 



.-id-- .f thi- -pai.-.- i- m-irly -traijht. and the two lower an-jle- rounded. Tie 'ater.il mar-in> 

 of - are u.-ual!y -lib parallel in th-- !-,w--r half, while in the upper half they approach 

 otln-r, and m----t above to form tin- upp-T an-l--. In >"iuc -p-'cimen- the- apace i- m 

 rian-ular. an-1 it Would aj.pi-ar, tl that it- f.-rm i> a little variable. The >n_'th 



Space i- about one third the whole length of tin- \-ntral vahe, and it- width at the l-.v 

 mar.in a little le,- than it- l--n-ih. Tin- lower mai-in i- -ituat,--! a little ab-'\-- a line drawn 



.h-- -hell at mid length. The lower three-fourths of the s] 'riated longitudinally, 



and di\id.--l into two e-jiial p-.rti->n- by an ob-cuiv m<-dian 



11 b -id--, at the I, a f the- furanr-n. there i- a -hurt, -tuiit t""tl ! 



plat.-- below the tc.-th --\t'-nd but a -h'-rt -li-tan--.- int-, th-- \i -,it_\. \\h-n 



-n-idenly much dimini-lp d in ln-i-jht. th--\ furm a I-.-A ridj.- along th-- upper mar-in uf 

 mu-i-ular space I In- upper part uf the mu-eiilar -|. IC< ifi d--ep!\ :nt-i the -ni 



uf th-- -hell, which i- \'iy thi--k and -oli-1 in ti d halt" illillin 



Th- r. i- H"t mii'-li tu add I" th-- al,-,\.- -jii-it-d full ile-cript i--n uf thi- c-'inm-.n (',. miter 



bj ^Ir Ilillin--; but ih--i !--w puint- >. Inch ma\ be n-it- d 



linguiform --\t--n-i-,n ->f th-- front of tin- ventral valve, and tin 



ily fnll\ developed in adult examples. 1 1- no- m 



tl.- -hell nriy exluhii n-ithin_- ni"i-- thin a har-l in-lul it in >i 



,-n. also proportionate!) more oircu : ' '* of 'in- \nti 



ii . but tin r-- i beneath tin- l ik \\ ln--h b\ th- 



im-ur'.ation of the 1,,-ak i>ntii.l in the p.-r d. ami \>hi--li --an - in 



the d- taehi-d \ al\e. 



1 ' "I the interior of th-- \-ntrd \al\-- of tin mm-'ii in ; 



