EXPLANATION OP THE PLATES. 591 



Fig. 2. Portion of a section taken through the inner whorls, transversely to the axis of 

 the spire ; showing the commencement of the canal-system in central lacunae 

 from which a spiral extension may be traced that gives off branches proceed- 

 ing into one of the spines; and showing the free inosculation of these 

 branches in the substance of the spine with others derived from the nearest 

 chambers, so that the entire substance of the spine is minutely intersected by 

 them. Magnified 50 diameters. 



Fig. 3. More enlarged portion of a section passing through the ' supplemental skeleton ' 

 in the immediate neighbourhood of the surface of one of the chambers; 

 showing the freedom with which it is traversed by the canals. Magnified 100 

 diameters. 



Fig. 4. Section of the disk taken through the later whorls, transversely to the axis of 

 the spire : a, central lacunae of the canal-system excavated in the portion of 

 the intermediate skeleton overlying the primordial chamber; a\ a 2 , a 3 , a*, 

 successive whorls ; b, b l , # 2 , the proper walls of the chambers of the last whorl, 

 receiving successive increments in thickness from exogenous deposit; c, c, 

 septal apertures ; d, d l , d 2 , d 3 , the ' supplemental skeleton ' of the last whorl, 

 showing its progressive increments in thickness ; e, e l , e*, e*, e', e?, the spines 

 in order of age, as determined by the portion of the supplemental skeleton 

 from which they are outgrowths. Magnified 50 diameters. 



Fig. 5. Section of a portion of the 'supplemental skeleton' taken near the surface of 

 the disk, in a direction transverse to the axis of the spire, showing the distri- 

 bution of the orifices of the canals around the non-canaliferous spaces e, e, e. 

 Magnified 100 diameters. 



Fig. 6. A young specimen of the hispid variety (see Plate XIX. figs. 8, 9). Magnified 

 30 diameters. 



Fig. 7. Section of a young Calcarina, taken transversely to the axis of the spire, showing 

 at a and elsewhere a diverging arrangement of the branches of the canal- 

 system analogous to that of Polystomella. Magnified 60 diameters. 



Fig. 8. Section of a young specimen of the hispid variety taken in the same direction. 

 Magnified 60 diameters. 



PLATE XXI. 



All the figures in this Plate refer to Tinqporus. 



Fig. 1. Fjxternal view of a conical specimen of T. lasvis, as seen laterally at a, and on its 



basal aspect at b. Magnified 25 diameters. 

 Fig. 2. Section of T. Icevis in a direction parallel to the base, showing an irregularly 



concentric arrangement of chambers, the floors of which are perforated with 



numerous foramina. Magnified 50 diameters. 



