LAMELLIBRANCHIATA. 281 



MODIOMORPHA ARCUATA. 



PLATK XXXVI, FIG. 21. 



ModimnorpJta arcuata. Halt.. Pal. N. Y., vol. v, pt. 1. Plates and Explanations: PI. 30, fig. 21. 1S83. 



Shell of medium size, obliquely sub-spatulate or narrowly and obliquely 

 elongate-ovate; length more than twice the height; basal margin arcuate, 

 the greatest concavity a little more than one-third of the length from the 

 anterior end ; post-basal extremity abruptly rounded and more gently recurv- 

 ing to the cardinal margin, which is nearly straight and extends for about 

 one-half the length of the shell. Anterior end produced, nasute, the margin 

 abruptly rounded, limited by a sinus which extends to the base, more than 

 one-third the length of the shell from the anterior end. 



Valves depressed convex in the posterior half, becoming gibbous and sub- 

 angular on the anterior half. 



Hinge-line oblique, straight,, extending for less than half the length of 

 the shell. 



Beaks small, sub-anterior, pointed, closely appressed and angular. Umbonal 

 region prominent, gibbous, sub-angular, the angularity extending for more 

 than two-thirds the length of the shell. 



Test thin, with fine, thread-like concentric striae. The striae coalesce, 

 scarcely forming fiiscicles along the base and cardinal slope. Anterior mus- 

 cular impression sub-circular, close within the anterior margin of the shell. 



The specimen described has a length of 66 mm., and a height of 25 mm. 



This species is narrower and more elongate than any other form of the genus, 

 and is only approached by some of the extreme varieties of M. concentrica. It 

 bears very little resemblance to any known species found in the rocks of this 

 age, and is an extremely rare form, a single specimen only having occurred in 

 all the collections made in this horizon. 



Formation and locality. In the harder arenaceous shales of the Hamilton 

 group at Bear's gulf, Schoharie county, N. Y. It is associated with Spirifera 

 mucronata, Rhynchonella congregata, and Chonetes coronata. 



