PLATE XVI. 



All the figures in this Plate illustrate the structure of Polystomella, and the same letters are used to 



mark corresponding parts throughout. 



a, (^, or. Chambers of successive whorls ; in figs. 7, 8, 9, consecutive segments of the same whorl. 

 b, h^, b^, 6'. Septa of alternating whorls. 



c, c, c, c. Septal apertures ; in figs. 7, 9, c' c', the stolons passing through those apertures. 

 d d! , d d' . Meridional canals. 



e, e. Spiral canals ; e e' , the same transversely divided. 

 /, /. Diverging canals. 

 9 o' 1 'J 9 • Surface of the last-formed portion of the spiral lamina, showing a row of furrows passing 

 across the septal bands, into which the diverging canals open in immediate contiguity to them. 

 h li , h li . Older portion of the spiral lamina, showing the replacement of each series of furrows by two 

 rows of punctations. 

 i i' , i i'. Surface of the interior whorl, showing the obliteration of the septal bands, and the rows of 



dimpled depressions into which the diverging canals open. 

 k, k, k. Retral prolongations of the segments ; in fig. 5, k', k', mark the cuh de sac in which they are 

 lodged in the shell of Pohjstomella crispa. 

 I I, I' I' . Exogenous deposit of shell filling up the depressions in the umbilical regions of both lateral 

 surfaces, and traversed by straight vertical canals. 



Fig. 



1. — Ideal representation of a specimen of Polysto- 

 mella craticuluta, laid open to show its internal 

 structure, and the umbilical deposit removed 

 from its upper surface to show the spiral 

 canal. 



2. — Polystomella craticulala, as viewed at a in its 

 lateral aspect, showing the septal ridges and 

 the intermediate double rows of punctations, 

 and the irregular distribution of similar punc- 

 tations over the umbilical deposit of exogenous 

 substance ; the same shown in front at b, so 

 as to bring into view the septal plane, and its 

 row of multiple apertures. 



3. — Vertical section of P. craticulala ; magnified 20 

 diameters. 



4. — Side view of P. crispa, showing the fossettes 

 along the anterior margin of each septal band ; 

 magnified 25 diameters. 



Fig. 



6.— 



Vertical section of P. crispa ; magnified 50 

 diameters. 



Segments from two consecutive whorls of the 

 animal body of P. crispa, showing their forms 

 and connections ; magnified 100 diameters. 



Portion of a cast of three adjacent segments of 

 the same whorl of P. craticulafa, showing 

 their forms and connections ; magnified 40 

 diameters. 



Fragment of P. craticulala, showing the furrowed 

 internal surface of three of the chambers, and 

 the relation of the furrows to the diverging 

 canals ; magnified 40 diameters. 



Internal cast of the chambers and canals of 

 Polystomella craticulala, representing the form 

 of the body and the distribution of the canal- 

 system ; magnified 40 diameters. 



