THE NAUTILUS. 21 



dinals is also characteristic of rubiginotus. In the remaining four of 

 the species above mentioned, the character and relative position of 

 the pseudo-cardinals are quite different, and are similar in all. 



Taking ohliqua as an example, the interdentum is very broad. 

 The posterior pseudo-cardinal of the left valve is low and broadly 

 triangular in shape and the blunt apex points obliquely forward and 

 not directly upward ; the anterior pseudo-cardinal is very low and 

 bends around the broad and comparatively shallow socket for the 

 opposing pseudo-cardinal. This socket is almost quadrate in shape, 

 extends obliquely forward and a line from the upper to the lower 

 corner is almost perpendicular. The pseudo-cardinal of the right 

 yalve is broadly triangular, and the anterior an*i posterior sides are 

 about equal. It is surrounded above by a " pit " which is very shal- 

 low in front of the tooth and longer and shallower behind the tooth 

 than in the trigonus group, triangular in shape and quite oblique. 



The whole effect of the hinge in this group is that all the teeth are 

 subparallel and project obliquely backwards from the beak ; while in 

 the trigona group the pseudo-cardinal and lateral teeth met at a de- 

 cided angle under the beak. 



This arrangement of the hinge in obliqua and its allies sensibly 

 affects the external form of the shell, so that there is seldom any 

 occasion for mistaking to which of the two groups any particular 

 shell belongs. 



It would seem clear, therefore, that there is no occasion for con- 

 founding the undatus of Barnes with any of the four species grouping 

 about obliqua and that in seeking to identify that species, obliqua and 

 its allies may be dismissed from further consideration. 



If this be conceded, then it follows that Lea's trigonus is either 

 identical with undatus or rubiginosus or is a district species from 

 either. That the trigonus of Lea is specifically distinct from his 

 rubiginosus does not require argument in spite of the tentative union 

 of the two species by the Western Academy of Nat. Sciences caused 

 no doubt by the uncharacteristic figure given by Lea of his type. 



Rubiginosus is well characterized by its more broadly triangular 

 or subquadrate and more compressed shape and less prominent beaks, 

 which are less incurved at the tips, and the lack of the pronounced 

 angle on the posterior ridge. 



The possibility that trigonus and rubiginosus can be specifically 

 identical may be consequently most emphatically answered in the 



