860 PROCEEDINGS OF THE NATIONAL MUSEUM. VOL.XX. 



MICROYOLDIA Verrill and Bush, 1897. 



Microyoldia VERRILL and BUSH, Amer. Journ. Sci., Ill, pp. 56, 62, January, 1897. 



Type. Microyoldia regular-is (Yerrill). 



Shell small, tightly closed, veneriform, with the anterior end shortest 

 and with the beaks turned forward. A posterior marginal ligament in 

 a distinct groove, continued under the beaks. Hinge-plate and teeth 

 rather strong; the anterior series of teeth the shorter, forming a 

 marked angle with the posterior series, llesilium supported by a rela- 

 tively large and strong chondrophore, placed on the surface of the 

 hinge plate, distinctly behind the beaks and at the proximal end of the 

 posterior series of teeth. Pallial line indistinct. 



The curious little shell for which this genus is proposed is remarkable 

 for its form and the size and position of the cartilage and chondro- 

 phore, as well as for its few blunt teeth. If we are correct in our con- 

 clusions as to the anterior and posterior ends, the beaks turn forward! 

 as in Tindaria. The principal reason for considering the longer end 

 posterior is the existence of a well-formed ligament and groove along 

 that end and not on the shorter one. 



MICROYOLDIA REGULARIS (Verrill). 

 (Plate LXXVIII, figs. 5, 6.) 



Yoldia regularis VERRILL, Trans. Conn. Acad., VI, pp. 228, 279, 1884. 

 Microyoldia regiilaris VERRILL and BUSH, Amer. Journ. Sci., Ill, p. 56, figs. 5,6, 

 January, 1897. 



This species closely resembles the very young of Megayoldia thracice- 

 formis (Storer) Verrill and Bush, in the character of the hinge. Speci- 

 mens of the latter measuring 3.5 mm. in length have the relatively 

 large, concave, cartilage plate just before the beaks, which curve 

 strongly backward and are nearer the center of the shell, and the teeth 

 are more numerous and more slender. 



In M. regularis the shell is cordate-ovate or veneriform. The beaks 

 curve strongly toward the short (anterior ?) end. There is on this end 

 a sunken luuular area defined by a slight groove which indents the 

 hinge-margin. The anterior ( *?) part of the hinge-margin is thickened 

 and incurved along the Inutile and bears an inner ridge and four or five, 

 small, blunt teeth of which the proximal two project above the margin 

 in a dorsal view, the others are low and rather obscure. Under the 

 beak the hinge-plate is thickened, sinuous, edentulous for a short dis- 

 tance; back (?) of this there is a large, thick, oblique, concave chon- 

 drophore which occupies the whole breadth of the hinge-margin and 

 projects inward beyond it as a shelf-like border; beyond this there is a 

 series of six or seven prominent, blunt teeth. The external ligament 

 lies in a distinct grove along a large part of the edge of the longer 

 (posterior?) dorsal margin and runs under the beak, but fades out in 

 front of it. The pallial sinus is not visible, consequently it is not 

 possible to decide which is the anterior end. 



