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EXPLANATION OF PLATE XXVIII. 



Fig. 1. Polystomella crispa ; magnified 70 diameters, its ordinary size being about ^ 

 of an inch. 



a. Small dorsal spines. 



b. Externally depressed spaces, the " fossettes " of M. D'Orbigny. 



c. Elevated folds, corresponding with internal grooves and their terminating 



tubes. 



d. Front septum closing in the first chamber. 



e. Row of oral (?) orifices, usually very indistinct, and often invisible. 

 /. Septa, separating the chambers. 



Fig. 2. Young state of the same, magnified 70 diameters. 



a. Translucent calcareous dorsal spines. 

 Fig. 3. Outer surface of part of the external wall of one chamber, exhibiting a sec- 

 tion parallel with, and near to the posterior septum; magnified 400 

 diameters. 



a. Elevated folds, corresponding with the internal grooves. 



b. Depressed spaces or fossettes. 



c. Tubular extremities of the internal grooves. 



d. Minute calcareous tubercles, shagreening the exterior of the shell. 



e. Internal plates, converting the grooves into tubes, posteriorly. 



Fig. 4. Portion of the external walls of two cells, viewed from the interior ; magni- 

 fied 400 diameters. 



a. Grooves corresponding to the elevated folds, fig* 1, c and 2 a, becoming 

 converted into tubes at d by the addition of the internal lamina, b. 



c. Part of the septum separating the cells. 



e. Patulous extremities of the tubes, as seen in a transparent specimen, 

 through the septum. 



/. External tubercles, seen where the section has cut through the deep por- 

 tions of the fossettes. 



g. The bases of the larger tubercles in front of each septum, as seen through 

 a transparent specimen. 



h. Minute foramina crowding the calcareous shell. 

 Fig. 5. Front view of an internal septum, the lateral parietes of the subsequent cell 

 being removed. 



a. Outer arch of depressions (not perforations) marking the position of the 



extremities of the external tubes. 



b. Inner arch of small oral (?) apertures, along the line of junction with the 



succeeding convolution, communicating between adjoining chambers, 

 and transmitting the connecting necks of the soft animal. 

 Fig. 6. Soft animal of Polystomella crispa, as seen after the calcareous shell has been 

 removed by the action of hydrochloric acid. The segments are drawn a 

 little further apart than is natural, in order to show their structure. 



