354 PALAEONTOLOGY OF NEW YORK. 



Surface marked by fine, regular, concentric striae. Interior unknown. 

 Three specimens measure respectively 10, 15 and 16 mm. in length, and 

 8, 11 and 12 mm. in height. 



This species resembles P. Knappianum in form, but has a much more distinct 

 sinus in the anterior portion of the valves, and so far as observed has main- 

 tained its small size, never exceeding 16 mm. in length in a collection of more 

 than thirty specimens. 



Formation and locality. In the lower part of the Chemung group, at Ithaca, N. Y . 



NY ASS A, Hall. 1870. 

 Nyassa arguta. 



PLATE LIII, FIGS. 7-20. 



Nyassa arffuta. Hall. Prelim. Notice Lamellibranchiata, 2, p. 28. 1870. 



" " Pal. N. Y., vol. V, pt. 1. Plates ami Kxplanations : PI. 53, figs. 9-20. 1883. 



Shell of medium to large size, elongate sub-elliptical or ovate-arcuate ; length 

 about twice the height ; basal margin nearly straight, often slightly arcuate 

 or contracted posterior to the middle, abruptly recurved at the post-inferior 

 extremity ; posterior margin curved or obliquely sub-truncate. Cardinal line 

 gently arcuate. Anterior end narrowed and abruptly rounded. 



Valves depressed below, gibbous above and in the umbonal region. 



Beaks sub-anterior, small, closely appressed, rising but little above the 

 hinge-line. Umbonal ridge prominent, rounded or sub-angular, arcuate, 

 extending to the post-inferior extremity ; below and parallel with the ridge 

 there is a flattening or depression of the shell extending from the beaks and 

 producing a constriction or sinuosity in the basal margin. 



Test thick, marked by lamellose concentric lines of growth, without other 

 ornamentation. 



The hinge is characterized by numerous small teeth or callosities beneath 

 the beak, which appear to be without special arrangement, the posterior ones 

 being directed backward and sometimes more elongated. Lateral teeth 

 two or three in number, parallel and extending nearly to the post-cardinal 



