EXPLANATION OF THE PLATES. 37 



PLATE II. 



Fig. 1. Side view of a characteristic specimen of Peneroplis : 1 a, front view, or septal 

 plane, of the same, showing its single row of isolated pores : 40 diam. 



Fig. 2. Front view of a young specimen, intermediate in its characters between Pene- 

 roplis and Dendritina : 40 diam. 



Fig. 3. Side view of a specimen of Peneroplis, of which the later chambers are widely 

 extending themselves laterally : 40 diam. 



Fig. 4. Septal plane of cylindrical prolongation ofSpirolina (see fig. 11), showing trans- 

 ition between the isolated pores of Peneroplis and the coalesced fissures of 

 Dendritina: 40 diam. 



Fig. 5. Side view of a specimen of Peneroplis, of which the later chambers are extending 

 themselves longitudinally : 40 diam. 



Fig. 6. Front view of a young specimen, intermediate in its characters between Pene- 

 roplis and Dendritina : 40 diam. 



Fig. 7. Side view of a specimen of Peneroplis, of which the later chambers are extending 

 themselves longitudinally, and of which the spire is more turgid, and the pores 

 arranged in a double row, as shown at 7 a : 40 diam. 



Figs. 8, 9, 10. Front views of young specimens of Peneroplis, showing various departures 

 from the normal type in the form of the septal plane and the disposition of 

 the apertures : 40 diam. 



Fig. 11. Side view of Spirolina, which bears the like relation to the Dendritina-type 

 shown in figs. 12 and 13, that the specimens represented in figs. 5 and 7 bear 

 to the ordinary Peneroplis-type ; the septal plane of its cylindrical extension, 

 with apertures of intermediate form between those of Peneroplis and of Den- 

 dritina, is shown at 11 a: 40 diam. 



Fig. 12. Side view of a typical specimen of Dendritina: at 12 a is shown the septal 

 plane and single large dendritic orifice of its last chamber, and at 12 b the 

 same, from the preceding whorl ; al, al, alar prolongations of the chambers, 

 which bear a much larger proportion to the principal cavity in the middle than 

 in the later part of the growth : 40 diam. 



Fig. 13. Side view of a specimen of Dendritina tending towards the Spirolina-type, the 

 last turn of the spire having already detached itself, and the last chamber 

 having completely lost its alar prolongations, as seen at 13 a: 40 diam. 



Fig. 14. Front view of a young specimen of Peneroplis, presenting a very marked 

 departure from the ordinary type in the form of its septal plane and the 

 disposition of its apertures : 40 diam. 



Fig. 15. Septal plane of a specimen resembling in general form that represented in 

 fig. 13, but showing a want of coalescence of the fissures of which the den- 

 dritic orifice is made up : the ridge-and-furrow arrangement of the walls of 



