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

 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 h'fe 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 spiral 

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

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

 subordinate value of plan of growth as a distinctive character. 



The author next proceeds to a like investigation of the genus 

 Alveolina, which he shows to bear a very marked resemblance to 

 Orbitolites and Orbiculina, in the simple concretionary texture of the 

 shell, in the freedom of communication everywhere existing among 

 the chambers, in the mutual relations of these to each other, and 

 in their mode of communication with the exterior ; whilst its plan 

 of growth is very different, the axis round which the spiral turns 

 being greatly elongated, and every additional whorl of the spire pro- 



