I N V ERTEBRATE PALJEONK >LO< '• V . 407 



I'mm a fragment of a large example; ami thai smaller specimens always have 



much loss complex lobes ami sinuses; while still larger ones may have them 

 even more complex. There arc also often considerable differences in the 

 details of the branches of the lobes and sinuses, even, in some instances, 

 between those of different septa of the same individual specimen. Sometimes 

 the septa are also more, and sometimes less crowded, even in specimens of 

 the same size. When much crowded, this causes differences in the details 

 of the branches, some of which are, as it were, pushed aside so as to give a 

 somewhat different general form to a lobe ; while in other cases one of the 

 branches may be almost entirely hidden, as seen at (.<) in the second lateral 

 lobe of the annexed cut. In the specimen from which this figure was made 

 out, this branch of the second lateral lobe, which I have restored in dotted 

 lines, has the appearance of being cut off, owing to crowding against one of 

 the branches of the second lateral sinus of the next septum behind it. 



On comparing authentic specimens from New Jersey with others of 

 nearly equal sizes from the Upper Missouri Cretaceous, they are found to 

 agree well in form as well as in all essential specific characters of the septa. 

 The New Jersey specimens generally have the septa less crowded, and the 

 lobes and sinuses proportionally somewhat shorter; but it is evident that no 

 specific, or even subspecific, distinctions can be based on such trivial differ- 

 ences. 



\\i this connection, it is perhaps hardly necessary to remark thai Dekay, 

 in originally describing this species from a New Jersey specimen, was mis- 

 taken in describing the siphuncle as being funnel-shaped, and " placed on the 

 margin nearest the center of the shell." It is evident that he mistook the 

 antisiphonal or inner central lobe for the siphuncle, which was probably not 

 exposed in his specimen. The fact that he also refers to figure 3, instead of 

 2, of his plate, in the Annals of the New York Lyceum, might also lead one 

 to doubt whether he intended the name A. placenta to apply to this species, 

 were it not for the fact that his description under this name was evidently 

 intended to apply to this shell, and not to his figure 3, which represents a 

 fragment that may belong to some other shell; while he refers to figure 2 

 in connection with his A. hippocrepis, the description of which clearly agrees 

 with figure 5 of his plate, and certainly not with figure 2. 



Dr. Stoliczka (in his Palseont. Indica, I, 90) places both A. placenta, 

 Dekay, and A. syrtalis, Morton, doubtfully in the synonymy of A. Guada- 



