From the Niagara Formation. 



71 



towards the front, presenting the feature of a broad, shallow, un- 

 defined sinus. Surface marked by strong radiating striae. 



The aspect of this fossil is characteristic of the genus Zygospira, 

 but the form is extremely gibbous and has the character of a ma- 

 ture shell, though its length is only two and a half milliemtres. 



Mekistella rectirostra n. sp. 



Shell broadly ovate, acute at the beak, thence rapidly expanding ; 

 the dorso-lateral margins nearly straight for one-third the length of 

 the shell, and then broadly curving, giving the body of the shell 

 (in older individuals) a depressed orbicular or sublentiform aspect : 

 younger shells ovate or acute. Valves almost equally convex. 

 Ventral valve most convex a little above the middle, thence curving 

 to the lateral and basal margins ; sinus none or very obscure ; beak 

 elevated, acute, attenuate, nearly straight or very slightly arcuate ; 

 area and foramen triangular without deltidial plates. Dorsal valve 

 very convex above the middle, sloping abruptly to the lateral, and 

 more gently to the anterior margin ; beak closely incurved into the 

 triangular foramen of the ventral valve. 



Surface usually appearing smooth, but is marked by extremely 

 fine radiating striae, which give a cancellated appearance under a 

 lens. The larger specimens measure eleven mm. in length by about 

 nine mm. in width. 



This species is very distinct from the two other forms of the genus 

 from this locality, both in its proportions and general aspect ; but 

 more especially in the nearly straight beak and open triangular fora- 

 men. The specimens have the aspect of Cryptoxella, but are clearly 

 shown to possess spires arranged as in Meristella. 



Spirieera bicostata ? var. petila. 



Shell obliquely subpyramidal, wider than long ; hinge-line 

 straight, about four-fifths the greatest width of the shell, rounded 

 at the extremities. Ventral valve gibbous, subpyramidal ; beak 

 incurved, umbo prominent, abruptly curving to the basal and baso- 

 lateral margins ; sinus visible at the apex and becoming wider and 

 deeper towards the front, margined on each side by a strong plica- 

 tion, and between this and the cardinal extremity on each side are 

 one or more obscure folds. Area elevated, triangular, margins 

 rounded ; foramen higher than wide, with elevated margins, and 



