335 



communication, and consequently the general plan of growth, have 

 a very considerable degree of constancy ; and altogether the ten- 

 dency is strongly manifested in this type to the greater individuali- 

 zation of the parts of the composite body, which in the preceding 

 must be looked upon rather as constituting one aggregate whole. 



In the present memoir this contrast is fully carried out by a de- 

 tailed comparison of two characteristic examples from these types 

 respectively, each of them having its own features of peculiar in- 

 terest. 



In Peneroplis we find, both as to the simplicity of the structure 

 of the shell, and the general disposition of the segments of the 

 animal, a close resemblance to the spiral forms of Orbiculina ; the 

 only difference being the absence of the transverse or secondary 

 divisions of the chambers. In what is considered its typical form, 

 the shell is a flattened spire, opening out widely in its last whorl ; 

 and the chambers communicate with each other (as does the last 

 chamber with the exterior) by single rows of isolated pores disposed 

 at regular intervals along the septa. But the spire is occasionally 

 found to be more turgid, and the rows of apertures to become 

 doubled ; and instead of opening out in the last whorl, it is fre- 

 quently prolonged in a rectilineal direction. In tropical seas there 

 are found minute shells resembling those of Peneroplis in their 

 very characteristic external markings, but having a very turgid 

 spire, and having the row of pores in each septum replaced by a 

 single large orifice with irregularly radiating prolongations. This 

 type of structure has been characterized by M. d'Orbigny as a 

 separate genus, under the name of Dendritina ; and when its spire, 

 as in many forms of Peneroplis, is continued rectilineally, it has 

 been distinguished as a third genus under the name Spirolina. The 

 author shows, by an extensive comparison of individuals, that the 

 single dendritic orifice is to be regarded as formed by the coalescence 

 of separate pores ; and that the extension of these into a single 

 line, or their aggregation into a cluster, is related to the form of 

 the septal plane, as determined by the degree of flattening or of 

 turgescence of the spire. Consequently in his view Dendritina and 

 Spirolina are but varieties of Peneroplis ; the former, which are by 

 far the largest and the most highly developed, being of tropical 

 growth, whilst the most flattened forms of the latter are the com- 



VOL. IX. 2 A 



