TJiv Lifc-Hisfnrii (if Sjiirifi'r hrvi--^. 143 



Size. — I'lic length of this metiian £Tvowth-line. in specimens 

 of ordinary size, is about live centimeters, or al)oiit two inclies. 

 As an example of shape. I Avill give tlie jiroportious of a medinm- 

 sized specimen (Xo. 260) of my own cabinet, whose outlines 

 are well i)reserYed : 



Median growth-line, - - - - - - 4.5 cm. 



Greatest breadth. - - - - - - 4.4^ " 



Total length of cardinal area, its extremities merely linear, 8.4 " 



Greatest elevation of shell above the plane of the margins, - 1.2 " 



In same plane, distance from hinge-area to extremity of beak, 0.6 " 

 Greatest separation of the two folds forming boundaries of the 



.sinus, -------- 1.4 " 



Beak. — The beak is prominent, and arches over the cardinal 

 area. 



('ardi)ial area.—TXiQ cardinal area is short and high; when 

 viewed ])erpendicnlarly to its surface, the convexity of the shell 

 above the beak appears about equal to the elevation of the area, 

 and it falls rapidl\' to a narrow linear area for the terminal jiart of 

 the hinge-line. 



Aperture. — The aperture is triangular, and in all perfect spec- 

 imens is found to be covered Avith a pseudo-deltidium. 



Pse II do-delti ilium. — This pseudo-deltidium is triangular, and 

 convexly arching outward and, in what seem to be normal s})ec- 

 imens, has the form of au equilateral triangle, though in other 

 specimens with short and very high area, the pseudo-deltidium 

 is narrow, forming an acute angle at the top. 



Surface. — The surface is in general smooth; only faint lines of 

 growth appear until near the margin, where the surface is often 

 coarsely imbricated by concentric lines of growth. 



Ohscure plications. — Hall mentions the fact of the obscure and 

 undefined radiating folds occurring in older shells. 



This is noticed to be a fact; and the only specimens in which 

 these obscure plications have been observed (by the w^'iter) are 

 those from the lowest layers in which the species occurs. Fur- 

 ther investigation may disprove the sujiposition, birt as far as 

 observation goes, the facts suggest that traces of radiating j^lica- 

 tions appear only on specimens from the lowest strata, i. e. at the 

 first appearance of the species, and only on the largest and hence 



