Dr. Carpenter on Peneroplis, Operculina, and Amphistegina. 291 



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 diiference 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 of 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- 

 paratively stunted inhabitants of the Mediterranean and other seas 

 of less elevated temperature. 



In Operculina, on the other hand, we find the shell presenting 

 the minutely tubular structure which was first shown by the author ' 

 to exist in Nummulites ; to which genus Operculina is so closely 

 allied in structure, that the only positive difference between them 

 seems to lie in the tendency of Operculina to open out widely in the 

 last whorl, whilst Nummulites (according to MM. d'Archiac and 

 Haime) tends to close in. The author minutely describes the struc- 

 ture of Operculina, which presents a very remarkable development 

 of the canaliferous system ; he also enters into a detailed inquiry 

 into the relation of the numerous strongly-marked varieties of form 

 which it presents, — a question of much importance in regard to the 

 value of the characters of the reputed species among Nummulites ; 

 and shows that the range of individual variation in form and surface- 



