PLATE XIX. 



Fig. I. rscitdomonotis Cf. Jiaunii. Left valve showing posterior 

 lobation and contortion of costre and striae. The lobe is not sepa- 

 rated from the rest of the shell very distinctly in most of the 

 specimens, being more of a prolongation of the postero-ventral 

 part backwards. Natural size. 



Fig. la. Interior of right valve of a specimen, in all probability 

 of this form, showing the very deep byssal sinus and the location 

 of the retroactor scars behind the byssal sinus instead of on the 

 ear above. It also shows the adductor scar and traces of radiating 

 ribs are visible on the interior. Natural size. 



Figs. lb and le. Outlines illustrating degrees of gibbosity of 

 these shells. 



Fig. id. Another specimen showing more clearly the radiating 

 striae and costae and possessing a more distinct posterior lobe. 



Figs, ic and if. Views from above of two specimens of this 

 shell showing the form of the umbo. 



Figs. 2 and 2a. Right and left valves of a specimen of 

 Pseudomonotis hawni (?) showing great spinosity and broad beaks 

 (beaks are crushed somewhat) and that the left valve had been 

 attached.' Byssal sinus indistinct. Natural size. 



Fig. 3. Surface markings of Pseudomonotis tenuisiriata showing 

 the lamellae extending farther down toward the ventral part on the 

 ridges than in the furrows, in marked contrast to certain other 

 not distantly related, genera of shells in which the lamellat^ in the 

 furrows are more produced than on the ridges. Highly magnified. 



