TOO THK NAUTII.rS. 



Lawrence Basins be much more completely studied, especially in 

 regions where the divide is narrow. Together with this we need to 

 obtain and study the fossil forms of the Tertiary and Pleistocene. 

 Thus and thus only can we get a much more accurate and detailed 

 knowledge of the effect of the Glacial Period on the distribution of 

 animals. 



Plate VII. External and internal views of fossil Unio crassidens 

 from Green Bay. 



University of Wisconsin, Zoological Laboratory, November 29, 190^. 



ON THE SPECIES OF MARTESIA OF THE EASTERN UNITED STATES. 



BY CHAKLES AV. JOHNSON 



Three species of the genus Martesia are found on the eastern coast 

 of the United States. They are more abundant south of Cape 

 Hatteras, becoming less common or rare to the northward. Like 

 most burrowing shells they are subject to considerable variation. 

 There is also a great difference in appearance between the young 

 and adult shell, the large anterior gape of the young being closed in 

 the adult by a calcareous deposit called the " callum " attached to 

 either valve and extending to the middle or lower edge of the valve. 



The shell has a large protoplax and a narrow elongated metaplax 

 and hypoplax ; mesoplax and siphonoplax wanting; valves with a 

 single radial sulcus. The species can readily be distinguished by the 

 form of the proto[)lax, which though showing slight variation, prob- 

 ably due to a favorable or unfavorable situs, is quite constant in its 

 general character. 



Martesia striata (Linn.). Fig. L 



Pholas striata Linn., Syst. Nat. 12 ed. 1111, 1767. 



Pholtts pusilla Linn., Syst. Nat. 12 ed. 1111, 1767. 



Pholas nana Pultney, Dorset. Cat. p. 27, 1799. 



Pholas falcata Wood, Gen. Conch, t. 16, f. 5-7, 181.5. 



Pholas clavata. Lam., Anim. s. Vert. V, p. 446, 1818. 



Pholas cotwides Fleming, Brit. Anim. p. 457, 182o. 



Pholas Hornheckii Orb., Historia Fis. Polit. y Nat. de la isla de 



