METAZOA MOLLUSC A. 251 



view of an internal cast), showing the long tube, the 

 slightly oblique sutures, and the flaring aperture. As 

 specimens are usually preserved the siphuncle appears to 

 be distinctly marginal, but Clarke has shown that this is 

 not the case, but that a narrow portion of the septum lies 

 between the siphonal funnel and the shell wall. This is 

 shown in fig. 8, which is the interior of a portion of the 

 shell showing the intra-marginal position of the siphonal 

 funnel. 



Mimoceras ( = Goniatites) compressum (PI. 600), has a 

 protoconch which is permanent throughout life. This is 

 well seen in figs. 1-3. These figures show that the shell 

 is nearly straight at first, and that the septa, and therefore 

 the sutures, are primitive and similar to those of the Nau- 

 tiloidea. The septa are, in fact, concave both in the 

 young and in the adult. The siphuncle is nearer the 

 center at first and later approaches the ventral side. 

 The adolescent shell is loosely coiled (fig. 3), the whorls 

 not being in contact. Later, however, the shell becomes 

 closely coiled, as seen in figs. 4, 5 ; also figs. 6, 7 (M. 

 ambigena). Fig. 5 shows the older septa with the 

 siphonal lobe. The outline of the aperture of the shell 

 is rounded on the dorsal as well as the ventral side (fig. 

 7 ). This is an important character showing the absence 

 of an impressed zone at a late stage. 



In these generalized forms of the Ammonoidea, signifi- 

 cantly called the Nautilinidae, the Nautiloid characters 

 are retained for a considerable length of time, but later 

 new structural features appear which are distinctly Am- 

 monoidai. 



Adult Goniatites in general have the septa concave as 

 in the Nautiloids, but the sutures though simple (No. 60 1, 

 Brancoceras ixion Hyatt), have a few lobes (backward 

 bendings of the septa) and saddles (forward bendings of 

 the same parts). The aperture is simple with ventral 

 sinus like that of Nautilus. The siphuncle is at first cen- 

 tral in position and later reaches the ventral margin where 



