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foration and the curvature of the first whorl is so uniform that its 

 early origin cannot reasonably be attributed to that as a cause. The 

 furrow deepens immediately and affects the outline of the fourth 

 suture. A slight dorsal lobe appears in the suture of the third 

 septum at the same time with the annular lobe, and is better 

 given in Fig. 33 than in Fig. 34. The flattening of the dorsum 

 is apparent in the second suture, and, so far as I could see after 

 repeated observations, my former figure in Embryology of Cephalo- 

 pods was erroneous in placing an annular lobe in this suture. This 

 species shows highly accelerated development in all of its charac- 

 teristics and this acceleration is obviously genetic and independent 

 of the size of the umbilical perforation, which is very large con- 

 sidering the fact that it is a Jurassic species. 



I have also examined another less perfect specimen of this species 

 having a considerable part of the shell preserved, but the first and 

 second apical chambers were lost. The external shell of the um- 

 bilical zones had longitudinal ridges as well as external parts of the 

 lateral zones and the venter in the paranepionic substage. The 

 form of the whorl in section near the ends of the paranepionic sub- 

 stage remains about the same, except that the venter becomes 

 slightly broader and flatter. The umbilical perforation is not quite 

 so large and the gyroceran bend is more abrupt in this specimen, 

 but otherwise it is exactly similar to the first described specimen. 

 It is in Museum of Boston Society of Natural History. 



Cenoceras clausum. 



Nautilus clausus, D'Orb. {Terr. Jur ass., PI. xxxiii). 



Loc, St. Vigor, near Bayeux, Inf. Oolite. 



PL xii, Figs. 12-15. 



This species has a small umbilical perforation. The form and 

 general aspect are very similar to those of other compressed shells 

 of this genus, but the shell in the paranepionic substage has peculi- 

 arly well-marked and broad growth bands with interrupted longitu- 

 dinal ridges. The ana- and metanepionic volutions are shown in 

 Figs. 13-15 and have a rounded dorsum, the dorsal furrow appears 

 in the paranepionic at the gyroceran bend and deepens rapidly as 

 the shell grows around the perforation. The amount of involution 

 is probably about the same as in Cenoceras granulosus, which it also 

 resembles in general aspect as well as in ornamentation. 



