NAUTILOIDEA. 



841 



which the shell (fig. 754) is more or less curved, the dorsal side 

 being convex and the ventral side concave, and the shell being thus 

 " endogastric ; " while the siphuncle runs on the ventral aspect. The 

 form of the aperture is not affected by this reversal of the curva- 

 ture of the shell, except that the smaller opening of the T-shaped 

 mouth, corresponding with the funnel of the animal, is now turned 

 towards the concave side of the shell. 



The genus Gomphoceras (including Phragmoceras) has been stated 

 to range from the Ordovician to the Carboniferous ; but, according 

 to Foord, the pre-Silurian and post-Silurian types referred to this 

 genus are of doubtful affinities, or are certainly known to be refer- 



Fig. 754. — Gomphoceras {Phragmoceras) ventricosum. Silurian. The right-hand 

 figure shows the form of the aperture. 



able to other genera. The genus is therefore restricted, so far as 

 clearly ascertained, to the Silurian period. 



Family 5. Ascoceratid^:. — In this family the shell, when fully 

 grown, is sac-like and truncated below, the body-chamber occupying 

 most of the ventral side, while the last few septa are specially modi- 

 fied. The aperture may be simple or contracted. 



The type of this family is the genus Ascoceras (fig. 755), which is 

 defined by Foord as follows : " The shell is of a sac-like form, 

 essentially straight, but always more convex on the ventral than on 

 the dorsal side. The apex is unknown, the shell being always 

 truncated. The transverse section may be elliptical or circular. 

 The body-chamber occupies nearly the whole length of the shell on 

 the ventral side, and contracts into a neck-like prolongation towards 

 the aperture, which is simple. The last few septa are abnormal ; 

 they have the usual shape and position on the ventral side of the 

 shell, but on the dorsal side they coalesce, and sweeping upwards 

 in a sigmoid curve (as seen in section, fig. 755, a) form a series of 



