280 PALEONTOLOGY OF NEW YORK. 



MODIOMOEPHA MACILENTA. 



PLATE XXXVII, FIG. 17; and PLATE XXXIX, FIOS. 1111. 



Uodimnorpha macileiita. Hall. Pi-elim. Notice Laiiiellibi-andiiata, 2, p. 7ti. 1870. 



" " •• Pal. N. Y., vol. V, pt. 1. Plates and Explanations: PI. 37, fig. 17; pi. M9, 



Rgs 17-21. 1883. 



Shell large, rhomboid-ovate ; length one-third greater than the height; basal 

 line nearly straight for more than half its length and abruptly curving both 

 anteriorly and posteriorly. Posterior end oblique, more gently curving from 

 the post-basal margin to the cardinal line, which is oblique and arcuate. An- 

 terior end short, rapidly declining and abruptly rounded below. 



Valves moderately convex ; greatest convexity in the umbonal region, 

 gradually merging into the general convexity of the valves. 



Hinge-line extending about half the length of the shell, slightly arcuate. 



Beaks small, sub-anterior, appressed, scarcely rising above the cardinal line. 



Test thin, marked by regular concentric striae which are scarcely fasciculate 

 on any portion of the shell. The surface shows the irregular vascular mark- 

 ings of other species of this genus, and of some of the species of Spiienotus. 



The anterior muscular impression is strong, situated a little anterior to 

 the beak and just within the margin of the valves. 



Three specimens have the following dimensions respectively : Length 90, 

 102, 108 mm.; height 60, 65 and 75 mm. 



All the individuals which have been recognized as of this species are of 

 comparatively large size, and bear considerable resemblance to the larger 

 individuals of M. alta, but may in general be distinguished by their less convex 

 valves, less defined umbonal ridge, usually more rounded posterior extremity, 

 and finer and more regular concentric striae. 



The typical forms here recorded under the specific designations of M. myliloides, 

 M. dlta and M. macilenta, are easily distinguishable ; but in the study of large 

 collections we find so many intermediate forms that it becomes difficult, if not 

 impriu!ticable, to arrange them under these several designations. 



Formation and localities. In the soft calcareous shales of the Hamilton group, 

 shores of Canandaigua lake. North Bristol, Ontario county; Bellona, Yates 

 county; and York, Livingston county, N. Y. 



