646 PROCEEDINGS OF THE NATIONAL MUSEUM. vol.50. 



lengths and irregular direction, but seldom do they inclose pits. In 

 a line 10 mm. long 8 to 10 ridges may be counted. The character of 

 this sculpture is best shown in figure 2, plate 35. 



Aspideretes lancemis is distinguished from all described species by 

 the ornamentation of the carapace, the presence of nine costals, and 

 by the fact that the seventh neural is in contact with the eighth pair 

 of costals. 



The notch on the median anterior end of the first neural shows that 

 there was a preneural, which indicates at once that the specimen can 

 not be assigned to the genus Amyda, but whether it should be referred 

 to Plastomenus or to Aspideretes is not so clear. The absence of 

 identifiable plastron bones makes it impossible to be sure of its 

 generic affinities, but since the genus Plastomenus has not yet been 

 recognized from the Lance formation I therefore refer it to the genus 

 Aspideretes, until such time as the discovery of more perfect material 

 will make it possible to definitely determine its generic designation. 



EXPLANATION OP PLATES. 



PlATE 32. 



Superior view of the carapace of Baena haiji. No. 6728, U.S.N.M. Type- 

 specimen, one-third natural size, 



Plate 33. 



Inferior view of the plastron of Bacna hayi. No. 6728, U.S.N.M. Type- 

 specimen, one-third natural size. 



Plate 34. 



Superior view of the carapace of Aspideretes lancensis. No. 6727, U.S.N.M. 

 Type-specimen, one-third natural size. c. 1, c. 8, costal plates one and eight; 

 c. 9, notch for the missing ninth costnls; n, rlj^ht end of the nuchal plate; pn, 

 notch on end of the first neural for the reception of the preneural. 



Plate 35. 



Fig. 1. Distal ends of tliird and fourtli costals of Aspideretes lancensis. No. 

 6727, U.S.N.INI. Type-spocimon, nuturnl size. Shows cliaracter of sculpture of 

 the carapace. 



Fig. 2. Fragment of plastral bone of above specimen. Shows character of the 

 sculpture of the plastron. Natural size. 



