I-OI'A 





casts, both from Mmn-.ii'oli-. tln-y are f.iintly indicated. Muscular scars not 

 observed.* Length of apcrtmv -_'n nun.: width of same 1:1. "> nun.: length of shell 

 from apex to pn-t.M ior margin _'<; mm . These dimensions in a large and a very 

 -mall -.pecimen are respectively al.out '.!'., 10 and 4'J mm.. and 1.".. :: and '>.' mm. 

 The large specimen has -.uilered from pressure so that the width is less than 

 normal. 



io from the fact that it grew to a much larger si/.c, this species resembles S. 

 rugosa var. / Walmtt (Tenth Ann. I.Vp. II. S. Geol. Surv., pi. LXXIV. figs. 5, 5a) 

 more closely than it does any Lower Silurian shell known to us. 



Formation and loea/tff. Stones Hlv.-r krn.up. Vaniixenila Ix-d. Minneapolis, Minnesota, and Ib-lolt, 

 WUoooftln. 



CbUMtioM. Geological and Natural History Survey nf Minnesota; University <>f Minnesota: Univ.T- 



\ M-nn-lll. 



Kfffitltr. ' 



suiMH \ i Ni.riKORMIS, ..>/). (f'lrii-h.) 

 ri.vTK I -. 



Sli.-ll nn^niform. acuminate-ovate from above, rounded posteriorly, narrow in 

 front when- the ai>\ curves more or less strongly downward, sometimes nearly to 

 the plane of the aperture and projects considerably beyond the anterior margin; 

 aperture horizontal, ovate, usually much more broadly rounded behind than in 

 front. Surface with distinctly elevated, regular, sublamelliform, concentric lines, 

 from 0.2 to 0.6- mm. apart, the distance between them increasing with growth; 

 crossing them very fine radiating lines; test rather thick; surface of cast smooth. 

 Three specimens have the following dimensions: length of aperture 6.5, 10.2 and 

 11 ."> mm.: wi. 1th of same 5.8, 9 ami 'U mm.; length of shell 7.7, 12.8 and 15 mm.; 

 hight of same 3, 4.5 and 6 mm. Old specimens have thick edges and are relatively 

 longer than medium and young examples. This is because the increase of the 

 shell, after a certain stage, takes place prim-i pally at the posterior border. For the 

 same reason the beak appears more strongly incurved in old shells. 



This species is clearly <listim-t from all previously .le-criliod patelloid shells. 

 Then> may be some doubt about the generic position, tho anterior hight being !>- 

 than it should be in a true v Tin' < 'amKrian >'. ' r/<n</r//a Walcott, however, 



exceeds our species in that ro-pt>ct. while in its younger stages it is decidedly like 

 some of the forms referred to S. rugosa by the same author. We believe, therefore. 

 that we cannot be far wrong in placing it un-icr 



Formation and lor<iltty. 1 \\-r U-<|.if tin- Tn-nton ifmuji. t.otw.-.-n Hurviti and Danville, K.-nt 

 ColUttirm.-K. O. rirlrh. 



Mr. Srdoa wy* of tbp tuuv-ular >n that UH>T " * n ' " the CMU bat tlwra >pp*n to b m row of 



boat M pMBln* nM*d tb tbvll from 3 to S mm. boT UM Up of MM *prrtur ' 



