270 Systematic Paleontology — Middle Devonian 



Description. — 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. Valves moderately convex; in old shells gibbous in the 

 umbonal region. Hinge-line oblique, extending to about the middle 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 sliells, 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 anterior 

 muscular impression is well marked and situated just within the anterior 

 margin below the beak. Length, from 50 to 91 mm. ; height, from 29 to 

 •50 mm. 



The Maryland Collection contains a left valve, from the east bank of 

 Evitts Creek below Wolfe Mill, from which the umbonal region is missing, 

 that has been referred to tliis species. It is smaller than the normal speci- 

 mens of this species and probably the shell had not reached maturity but 

 the proportions of length and height agree witla those of this species. The 

 slope from the umbonal ridge to the cardinal line is more marked than in 

 M. mytiloides and resembles more nearly that of AI. subalata but the con- 

 centric striae and the marked contraction of the anterior end are distinc- 

 tive characters of the former species. Another specimen of apparently an 

 immature right valve of this species was found in western Maryland, the 

 exact locality of which is unknown. 



Length, 36 mm. ; height, 21 mm. 



Occurrence. — Eomney Formation, Hamilton Member. East bank 

 Evitts Creek below Wolfe Mill; on Hancock-Harrisonville Eoad about 2 

 miles north of Hancock. 



Collection. — Maryland Geological Survey. 



