726 SYSTEMATIC PALEONTOLOGY 



being in some instances almost obsolete. Mr. Morton's type specimen, 

 which I have not seen, seems to have been very small, and to have had the 

 anterior end rounded from below, while Mr. Conrad's type of P. pectorosa 

 is full and round below and sloping above, while a cast of a single valve 

 which is figured appears to have been quite sharply truncate in front and 

 angular on the umbonal ridge. There is also much difference in the pro- 

 portional strength of the two sets of ribs in the different examples."- 

 Whitfield, 1885. 



Morton's type of P. cithara has, apparently, been lost and his descrip- 

 tion is so meager and his figure so inadequate that it is impossible to deter- 

 mine with absolute assurance its relationship to the P. pectorosa of Con- 

 rad. Conrad's type now in the Philadelphia Academy of Natural Sciences 

 is from the Monmouth at Tinton Falls, New Jersey, and there is no doubt 

 whatever about the specific identity of this form and the casts from the 

 Monmouth of Prince George's County. It seems not at all improbable, 

 though by no means established, that Conrad's species is distinct from 

 Morton's and possibly its descendant, since in Xew Jersey and Maryland 

 the former is restricted apparently to the Monmouth, the latter to the 

 Matawan. 



Occurrence. MONMOUTH FORMATION. Brightseat, Brooks estate near 

 Seat Pleasant, Prince George's County. 



Collections. Maryland Geological Survey, Philadelphia Academy of 

 Natural Sciences, New Jersey Geological Survey, U. S. National Museum. 



Outside Distribution. Monmouth Formation. Tinton beds, New 

 Jersey. Ripley Formation. Exogyra costata zone, ? Chickasaw, ? Union, 

 and ? Tippah counties, Mississippi. 



Genus MARTESIA Leach 

 [Blainville, Man. Mai., vol. i, 1825, p. 632] 



Type. Pholas clavata Lamarck = Pliolas striata Linne. 



Shell ovate-oblong, cuneiform, strengthened by three accessory plates; 

 young forms gaping anteriorly; valves closed at the completion of the 

 burrow with a calcareous septa or " callum " ; surface deeply sculptured 

 by a single radial sulcus. 



Etymology: Unknown. 



