151 



VIII. — Or Dimorphism in the Recent Foraminifer, Alveolina 



boscii Defr. sp. 



By Fkedekick Chapman, A.L.S., F.R.M.S., 

 Palaeontologist to the National Museum, Melbourne. 



(Bead February 19, 1908.) 



Plates II. and III. 



Preliminary Remarks. — The spindle-shaped tests of Alveolina 

 boscii will be familiar to all who have examined dredgings from 

 moderately shallow water in tropical regions. In the fossil state, 

 species of the same genus are found in Cretaceous, Eocene, and 

 Miocene limestones in various parts of the world. 



With regard to the occurrence of dimorphism in this genus — 

 the phenomenon of the two stages in the life-history of the organism, 

 in which the shell commences either with a large central chamber 

 (form A), or a small one (form B) — our knowledge is limited to one 

 instance, for the form B seems only to have been noticed, by 

 Munier Chalmas, in a fossil species.* In that example the micro- 

 spheric form was distinguished by a very small central chamber, 

 surrounded by five simple chambers, which were not subdivided. 



Occurrence and Description. — The usual form of the test in 

 Alveolina boscii, as found in our coral beach sands and shallow 

 water dredgings, is that having a comparatively short fusiform 

 shell with a large central chamber. 



It has lately been my good fortune to meet with the form B of 

 this species in some material kindly handed to me by Messrs. 

 Charles Hedley, F.L.S., and C. J. Gabriel, who dredged it from the 

 Great Barrier Eeef, at Cairns Keef, near the Hope Islands, Queens- 

 land. These dredgings consisted mainly of large foraminiferal 

 tests beloDging to the genera Orbitolites (0. eomplanata, Lam.), 

 Alveolina (A. boscii, Defr. sp.), Polystomella (P. craticulata, F. and 

 M. sp.), and Polytrema (P. miniaceum, L. sp.). The Alveolinm 

 were nearly all of the usual type (form A), but a few exceptions 

 occurred in which the test was of extraordinary length. Since the 

 microspheric shell is generally larger than the megalospheric, it 

 seemed highly probable that at last we had met with examples 



* Schlumberger, Ch., " Sur le Biloculina depressa d'Orb., au point de vue du 

 dimorphisme des Foraminiferes." Assoc. Franc, pour l'Avan. des Sciences, 

 Congres de Rouen, 1883, p. 526. See also Lister, in Ray Lankester's Treatise of 

 Zoology, pt. i. 1903, p. 111. 



[I am indebted to my friend, Mr. F. W. Millett, for a copy of this paper, which 

 does not appear to be in any of the Melbourne Libraries.] 



