PLATE IV F. 



Legend. K. Foi-amen. i t o(<.,.„i , ,»ii e .\ 



V Pediule-area Lateral walls of Hi.- -roove. 



,}/ I^li, M^?'; "■■ Anterior muscular n,|.r,.s. 



p'. Areal oa llos.ty ,. Median septum, 



g. Base of the externa pedicle-grroove. v. Vascular sinuses. 

 g'. Base ot the mternal pedicle-groove. 



DISCINISCA, D.vM.. 



Page 1-20. 

 DlSClNISlC.\ LAMELLOSA, Jilodeiil). 

 Rg. 1. '^he internal pedn^Ie-area enlarged to three dian.eters from the specimen represented in Plate 

 IV E, hg. 2. The pedicle-area IS a .somewhat thickened elevation, corres.M n.lin- to the d^.n 

 depression on the external surlace of the valve. The foramen transects the ar 'a^d re ■ Iv in 

 between it and the posterior margin is a narrow translucent huiiina, across which tie con c^nHc 

 ornamentation of the valve doe.s not pass. (-oncenmc 



SCHIZOTRETA, Kutorga 



^ Page 1-20. 



UBICULOIDEA (SCHIZOTRETA?) TENUILAMELL.\TA Hall 

 Fig. 2. The exterior pedicle-area of a specimen in which the groove is extremely sl.ort X 5 

 Fig. 3. A similar view of a specimen in which the groove is rel.Htivelv lar-rer, and the ha«^ walk -md 

 foramen well defaned. X 5. . ^ - ' ><i.., wans .inn 



Kg. 4. The corresponding parts in another specimen. X 5. 



Fig. 5. The groove of the same ; showing (hat the concentric growth-lines are retained on the broad lat- 

 Fi.. 'fi Thf ;yf «' ^^'^■'V'^'^ \^"r "^.™'°'.'"^ """fe'h less distinctly delined than in some instances X 1. 

 Fig. 6. The pedicle-charactei-s ot the interior, drawn from a natural cast of the exterior, in wW h fro „ 



Ihe oST"" " """■ "" "'"■'"' '^'"'■"^'"" '"" •""'^^'•^•^^^ *" continuity whhthol; of 



Niagara limestone. Hamilton, Ontario. 



SCHIZOTKETA CONICA, Dwigllt. 

 Fig. 7. The^external characters of the pedicle-area; from the specimen represented in Plate IV e, fig. 7. 



ORBICULOIDEA, D'ORBio.Nr. 



Page liO. 

 . Urbiculoidea PULCHRA, .sp. llov. 



Fig. 8. The internal pe,iirle-area The elevated median i-idge corresponding to the base of the external 

 groove. IS slightly broken, a crack extending to the foramen. About the tnte°l moietv o f 

 Vi a n,,""^"^"*^""^ 'literal callosities have just be-un to form. X3 ei loi moieij ol 



t^g. 9. I he same, in a more advanced .stage of development. The callosities have increased to such a 

 degree |is to have almost come into contact for the entire le.gth of the area, lea Wnl- the nosi 

 tion of the internal ridge indicated by the groove between their edges. X 3 ° ''""' 



„. Cuyahoga shales. Bma. Ohio, 



tig. 10. I he same parts, sti^ll showing a remnant of the groov=e formed by the meeting of the lateral cnl- 



losilies; also indicating the position of the fo'aminal openino- X 3 

 „. ,, ,0 rr.u Cuyahoga shales. Bacon.'iherg, Ohio. 



J<igs. 11, 1.. The same features, from individuals in which the lateral callosities have coalesced, leavinu- the 

 W T. rrJ^'r^l^^^.'' "Pe",' l^"t Otherwise concealing the structure of the area. X 3. ^ 



=■ ''• ''hig t^ani zz^^::::"^^^"" '^•■^='"^' ^-"^'^-^ = ^•^'^"•-^ ••- ^'-" -'i-" -i-'-- ^-^^ 



„. .,„ . Cuyahoga shales. Bprea, Ohio. 



Fig. 30. An internal cast of a brachi^al valve; showing the impressions of two strong, diverging vascular 

 sinuses (?) and finer markings about tlie margin. X1.5. 

 Cuyahoga shales. Ni-wnrk. Ohio. 

 Mg. i:. A very young pedicle-valve with a length of 2.2.5 mm., having the posterior margins of the 

 aperture unclased, while the i.edicle-groove has nearlv its entire normal length X 2 

 Cuyahoga shales. Berea, Ohio. ' 5 ^ • 



Orbiculoidea. coinp.iie O. pllchka. 



I'lgs. 14, lo, 16 The internal characters of the pedicle-area; showing the envelopment of th,. ri,ige bv the 

 lateral ciiHosities, m various stages of advancement. X :<• 

 Waverly group. Meadvilk, Pa. 



,,. ,. „,, . OrIUCULOIDEA .MARGI.XAl.IS, Wllil licld. 



°' ^'i^' ■"''^™«1 -characters of the pedi.-le-area. The callosities have assumed an irregular, hur l,i- 



l.iteially symmetrical form, almost enveloping the pedicle-rid. -e. x 3 

 Hamilton group, rttimil'ill". TVi.<:. ' " 



