LAMELLIBRANCHIA TA . 
277 
Modiomorpha mytiloides. 
PLATE XXXVII, FIG. 3; AND PLATE XXXVIII, FIGS. 1-16. 
Cypricardites mytiloides, Conrad. Geol. Surv. N. Y., Ami. Rep., p. 52. 1S41. 
Modiomorpha complanata , Hall. In error. Pal. N. Y., vol. v, pt. 1. Plates and Explanations: PI. 3S, 
fig's. 1-16. 1883. 
Modiomorpha planulata, “ Prelim. Notice Lamellibranchiata, 2, p. 74. 1870. 
Shell larger than the medium size, rhomboid-ovate, oblique; length less than 
twice the height; basal margin nearly straight, or very slightly concave 
anterior to the middle, curving to the anterior and posterior extremities; 
posterior margin abruptly curving below and more gently recurving toward 
the cardinal line ; cardinal margin arcuate. Anterior end narrow, extended, 
abruptly curved on the margin; somewhat defined by the sinus which extends 
from anterior to the beak to the middle of the shell. 
Yalves moderately convex; in old shells gibbous in the umbonal region. 
H inge-line oblique, extending to about the middle of the length of the shell. 
Beaks appressed, situated a little more than one-fourth the length of the 
shell from the anterior end. Umbonal region not defined; convex in young 
shells, becoming more gibbous in older individuals. 
Test of moderate thickness, concentrically striated with irregular lines of 
growth which are sometimes elevated into concentric ridges. The post- 
cardinal slope in well-preserved specimens shows fine vascular markings 
similar to those on Splienotus solenoides. 
The anterior muscular impression is well marked and situated just within 
the anterior margin below the beak. Other interior characters unknown. 
The specimen fig. 13 of plate xxxviii preserves the ligament connecting 
the two valves. 
Three characteristic specimens have the following dimensions: Length 50, 
60, 91 mm.; height 29, 37 and 50 mm. respectively. 
The individuals of this species from medium size upward resemble M. cnn- 
centrica in form, but the anterior end is more produced and they do not show 
the regular sharp concentric striae which mark that species. It also resembles 
some forms which are referred to M. alta, but may be distinguished by its more 
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