GENUS PENEROPLIS. 



89 



and that its continued increase upon the plan which has ah-eady begun to manifest itself 

 would convert it into a Spirolina. On the other hand, the Spirolina represented in fig. 4 

 has, up to the time of the substitution of the rectilineal for the spiral mode of growth, all the 

 characters of an ordinary BendnHna.* Thirdly, the differences of general configuration 

 between Peneroplis and JDendritina are differences of degree, and present themselves in 

 very variable amount in different individuals. For, starting from those forms of Peneroplis 

 in which the spire is least compressed, the transition is easy to those Bendritince whose spire 

 is least turgid, and whose septal plane is not broader in proportion to its length than it often 

 is in Peneroplis. From the most compressed forms of Bendritina to those which have the 

 most turgid spires and the widest septal planes the gradation is insensible, scarcely any two 

 individuals according in their proportions; thus we find that whilst the septal plane is 

 sagittate in some (Fig. XX, A, c), it tends to become reniform in others (b, d), the margin of 



P'IG. XX. 



B c 



Front views of four specimens of Dendritina. 



the spire, which is almost carinated in the first case, becoming obtuse and even crescentic in 

 the second. Again, the extent of the investment of the earlier whorls by the latter varies as 

 much as the degree of turgidity; forwhilst in some of the most compressed forms, as in Peneroplis, 

 each whorl does little more than apply itself to the margin of the preceding (Fig. XXI, a, c), 

 the more turgid the spire becomes the more completely (generally speaking) does it embrace 

 the preceding, the alar prolongations of the chambers thus coming to bear a large proportion 

 to their principal cavity (Fig. XXI, B, d). 



129. Still it may be said that, notwithstanding all these points of resemblance, the 

 difference between Peneroplis and Bendritina is clearly marked out by the difference in the 



* The elegant crozier-like form of Spirolina is sometimes rudely imitated by Lituola , aud 



specimens of the latter, the Sp. agglutinans of D'Orbigny (lxxiii, p. 137) for example, have not unfre- 



quently been described under the former designation, although the texture of the shell, which is never 



arenaceous in Spirolina, and always arenaceous in Lituola, aftbrds a ready aud certain means of 



discriminating them. 



12 



