12 THE ROYAL SOCIETY OF CANADA 



The fifth is broadest behind with one postero-lateral short side, the 

 sixth is broadest in front where there is a short antero-lateral side. 

 The seventh neural is irregularly shield-shaped, with the pointed 

 end behind. The sizes of these bones are given in mm. in the accom- 

 panying table: — 



Bone. Length. Breadth. 



Preneural 



Neural 1 



Neural 2 



Neural 3 



Neural 4 



Neural 5 



Neural 6 



Neural 7 



Of the costal bones the first is the broadest (fore and aft) at 

 mid-length The second, third, fourth, and fifth have about the 

 same breadth at mid-length, the second and fifth attaining a greater 

 breadth distally than the other two. The eighth pair meet along 

 the mid-line for a distance of 19 mm., and have a posterior free border 

 which is considerably longer than their breadth (fore and aft). The 

 seventh pair of costals meet at the mid-line behind the seventh neural. 

 The preneural and the anterior two-thirds of the first neural lie 

 between the first pair of costals. 



The sculpture of the carapace of this species consists of a shallowly 

 pitted surface in the neurals and at the inner end of the costals. 

 Passing outward on the costals the pits become larger, and lengthened 

 in a fore and aft direction by coalescence. Toward the distal end 

 of the bones the coalescence becomes more pronounced and results 

 in tortuous grooves of varying length, transverse to the length 

 of the bone The grooves are separated by narrow, frequently 

 inosculating, sinuous ridges whose breadth is much less than the 

 width of the enclosed sunken areas. 



The presence of the preneural bone, in combination with its 

 other structural characters, .clearly indicates the generic relationship 

 of this specimen. In no described species of the genus Aspideretes 

 is there the same assemblage of characters as is found in the outline of 

 the carapace, the size, shape and proportions of the seven neural 

 bones, the nuchal bone, and the posterior costals of this specimen. 



In some forms of Amyda the sculpture of the carapace closely 

 resembles that of certain species of Aspideretes. Generally the 



