Dr. W. B. Carpenter's Researches on the Foraminifera. 835 



present to the systematist, the author in this memoir details the 

 results of his investigations on the genera Orbiculina, AlveolinUt 

 C^jcloclypeuSy and Heterostegina, 



The genus Orbiculina has long been known, through its preva- 

 lence in the West Indian seas, which causes its shells to abound in 

 the shore-sands of many of the islands of that region. These shells 

 present great varieties of form, and have been ranked under three 

 distinct species ; but M. d'Orbigny has correctly inferred, from a 

 comparison of a large number of specimens, that their diversities of 

 form are partly attributable to differences in the stage of growth, 

 and partly to individual variation, so that all the OrbiciUince of 

 Cuba, the Antilles, &c., are referable to but one specific type. Of 

 the essential features of its structure, however, he would seem to be 

 quite ignorant ; since he ranges Orbiculina in a distinct order from 

 Orbitolites, to which it is very closely allied. This alliance was 

 first pointed out by Prof. Williamson, whose account of the structure 

 of Orbiculina, though defective and erroneous in certain points, is 

 nevertheless correct in the main. 



The author has had the opportunity of examining not merely a 

 considerable number of West Indian specimens, but also a set ol 

 specimens peculiarly remarkable for their high development, which 

 form part of Mr. Cuming's Philippine collection. Many of these 

 present the form of flattened disks, marked with concentric circles, 

 and having one or more rows of pores at their edges, not distin- 

 guishable, save by their prominent central nuclei, from certain forms 

 of Orbitolites formerly described. The similarity is equally great 

 in their internal structure ; so that, if a marginal fragment only 

 were submitted to examination, it would not be possible to say with 

 certainty whether it belonged to an Orbitolites or an Orbiculina, 

 The distinguishing character of the latter is derived from its early 

 mode of growth, which is uniformly spiral ; and from the circum- 

 stance that each of the first three or four turns of the spire not 

 merely surrounds, but invests its predecessor, thereby producing an 

 excess in the thickness of the earlier over that of the later-formed 

 portion, which gives rise to the central protuberance already men- 

 tioned. The transition from the spiral to the cyclical mode of in- 

 crease is effected (just as it is in those individuals of Orbitolites 

 which begin life upon the spiral type) by the opening-out of the 

 mouth of the spire, which extends itself on either side around the 

 previously-formed body, until its two divisions meet on the opposite 

 side, where they coalesce so as to constitute a complete annulus. 

 This transition may take place at any period of growth after the 

 completion of the first four or five turns of the spire ; so that we 

 sometimes meet with small specimens which have already become 

 discoidal and taken-on the cyclical plan of growth, whilst we occa- 

 sionally meet with full-grown specimens which retain the spira. 

 form, and show no tendency whatever towards the assumption of tiie 

 cyclical plan of growth. These facts obviously point to the very 

 subordinate value oi plan of growth as a distinctive character. 



The author next proceeds to a like investigation of the genus 



