100 THE NAUTILI'S. 



Lawrence Basins be much more completely studied, especially in 

 regions where the divide is narrow. Together with tin's 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 VH. External and internal views of fossil Utrio crassidens 

 from Green Bay. 



University of Wisconsin, Zoological Laboratory, November 29, 1904- 



ON THE SPECIES OF MARTESIA OF THE EASTERN UNITED STATES 



BY CHARLES W. 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 protoplax, 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. 1. 



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



Pholas pitsilla 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, 1815. 



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



Pholas conoiffes Fleming, Brit. Anim. p. 457, 1825. 



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



