106 THE NAUTILUS. 



part is thicker, and grooved, "sulcate", to bifid or bifurcate. 

 In fact, it is complex from its early nepionic formation, even 

 when quite small (see pi. 3, figs. 11, 12). 



Aside from that feature, which will have to be considered 

 later, C3 shows considerable differences as to shape. In some 

 species with thin shells and slight hinges, apparently more 

 primitive, it is straight or nearly so, longitudinal, with its 

 posterior end not or little thickened, barely or not complex. 

 But in most, the anterior and posterior parts, especially the 

 latter, are more or less curved downward, and generally the 

 posterior is more or less complex, as in pi. 3, figs. 1, 4, 6, 8). 

 The extreme of this formation is reached in P. amnicum and 

 dubium, when C3 is horseshoe or A-shaped. These species 

 have another feature which may be worth mentioning: the 

 two shanks or rand of C3 show a slight but distinct forward 

 direction in their lower parts. That is evidently caused by 

 the growth and shape of the mussel: its anterior part be- 

 comes much larger than the posterior and directed downward, 

 the dorso- ventral axis forms a curve, and in concordance with 

 that, the cardinals, C3 and 02, also grow obliquely. 



THE LEFT CARDINALS. The anterior, 02, fitting in below 

 C3 in the closed mussel, is generally of similar shape, except 

 that it is simple. Its base is straight, or more or less curved, 

 and then its lower face is more or less concave. The crest is 

 rounded or more often pointed, and from being bent more or 

 less upward, appears to be massive in lateral view. Its posi- 

 tion shows marked and significant differences : in some 

 species it is on the edge of the hinge plate and often even 

 more or less projecting downward over it, while in others it 

 is rather high up on the plate, which then is usually broader. 



The left anterior, C4, near the anterior end of the ligament 

 in its upper part, and adjacent to the posterior part of C3, 

 is usually more or less oblique and curved, its edge straight 

 (truncate) to somewhat rounded. In species with C3 and 02 

 straight and longitudinal, 04 is usually conform and parallel 

 with them. Generally its anterior (upper) part passes for- 

 ward above part of 02, and sometimes its whole length. 

 And very often it becomes connate with the end of the an- 



