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Plate XII. 



Figs. I and 2, Diorugoceras (Naut.) planidorsattwi, after Portlock, Geol. 

 Rep. Londonderry , etc., PI. xxxv, Fig. I ; Carboniferous. Fig. I, side view, re- 

 duced considerably. Fig. 2, section of outer whorl showing how completely the 

 contact furrow is moulded to the form of the whorl. 



Figs. 3-5. Pleuronautilus superbus, after Mojsisovics, Das Gebirge um Hall- 

 stadt, i, PI. xviii, Trias. Fig. 3, reduced one-third, showing the striated nepionic 

 stage, the very large umbilical peiforation, free ananepionic substage, also the 

 close-coiled stages. Figs. 4 and 5, lateral, dorsal views of the fragment of what 

 is probably another species reduced one-third, showing an impressed zone as 

 described and figured by Mojsisovics.* 



Figs. 6-1 1. Digonioceras rotundum, Oolite; Coll. Mus. Comp. Zoology; no 

 locality. 



Figs. 6 and 7, side and front views of nepionic stage and ananeanic substage, 

 natural size, showing especially the large, pear-shaped umbilical perforation and 

 the amount of involution which covers only about one half of the side. The 

 outer part of Fig. 6 still has remnants of the dorsal shell of the enveloping neanic 

 and ephebic whorl and parts of four septa clinging to it. Two older sutures in 

 the paranepionic volution and two younger in the metanepionic volution are also 

 shown and first to the third are seen in Fig. 7, which also shows the narrow ana- 

 and metanepionic substages. Fig 8, the same broken down, showing the apical 

 chamber and caeum enlarged in Fig. 9. Fig. 10, section of ananepionic substage 

 at first septum. Fig. n, section of metanepionic passing apicad of second septum 

 and showing early beginning of impressed zone, both have the venter down- 

 wards. N 



Fig. 12. Cenoceras (Naut.) clausum, after Barrande and Hyatt, Syst. Sil., 

 PI. cccclxxxix ; D'Orb. Coll. Jardin des Plantes ; Oolite; somewhat enlarged, 

 shows the well-defined impressed zone in the paranepionic substage and the first 

 three sutures. 



Figs. 13-15. Cenoceras (Naut.) clausum (?), St. Vigor le Grand near Bayeux, 

 Bronn Coll. Mus. Comp. Zool.; Lower Oolite. Fig. 13, somewhat enlarged, 

 showing the shape of umbilical perforation and ana-, meta- and part of parane- 

 pionic substages. The amount of involution is shown by the dark line. Fig. 14, 

 front view of same, natural size. Fig. 15, enlarged to show shape of ananepionic 

 substage at first septum, the venter down. 



Figs. 16-21. Cymatoceras (JVauL) (sp. ?), Texas. Fig. 16, side view of ne- 

 pionic stage, and ananeanic substage with sutures enlarged two diameters. The 

 approximation of the fourth and fifth sutures appeared in several specimens of the 

 young of this species. The third and fourth sutures are too close together in this 

 drawing and the spaces between the fifth, sixth, seventh and eighth, somewhat too 

 wide. The spaces between these last are not so wide as that between the third 

 and fourth in the original, this space being also slightly less than that between the 

 second and third. Fig. 17, section of apex and ananeanic substage enlarged two 

 diameters with the deep zone of contact. Figs. 18 and 19, the appearance of the 



* See p. 547, above. 



