GENUS AMPHTSTEGINA : — ^HISTOEY. 
31 
forms of the order Helicostegues, it was considered by M. d’Oebigny to depart from them 
in a character which he regarded of such fundamental importance as to serve as the basis 
of a distinct Order, that of Entomostegues ; of which the following is his most recent 
definition*: — “Animal compose de segments alternes, formant une spirale. Coquille 
composee de loges empilees ou superposees sur deux axes alternant entre elles, et 
s’enroulant en spirale.” The mode of increase of these shells, he elsewhere says f , presents 
a singular mixture of that of the Enallostegues with alternating chambers, and of the 
spiral involution of the Helicostegues. I have already had occasion 111) to point but, 
that in placing Heterostegina in this order, M. d’Oebignt has misconceived the structure 
of that genus ; and I shall now have occasion to show that he has fallen into an equally 
grave error in regard to Amphistegina. His definitions of that genus in the ‘ Forami- 
niferes Fossiles de Vienne ’ and in the ‘ Cours Elementaire de Paleontologie,’ are by 
no means accordant with each other : I take the last as the most authoritative. — “ Co- 
quille deprimee a spire embrassante, pourvue de loges alternes d’une cote et non de 
I’autre, separees, interieurement, par des cloisons longitudinales J.” 
165. The only inquiry yet made, so far as I am aware, into the minute structure of the 
shells of this type, is that of Professor W. C. Williamson, in the memoir § to which I have 
already had such frequent occasion to refer. His investigations were made on small spe- 
cimens of the AmpMstegina gibhosa, a species which seems to be pretty generally difiused 
through the tropical ocean, and which has been recently dredged up in great abundance 
by Mr. M'Andeew in the neighbourhood of Teneriffe. Professor Williamson showed 
by means of horizontal and vertical sections that Amphistegina has the general structure 
of Eummidites, but with this marked difierence (as he considered), that the shell is 
ineguilateral ; the spire making its convolutions obliquely instead of revolving in the 
same plane (in other words, being turhinoid instead of nautiloid), and the alar prolon- 
gations of the chambers being much larger, and extending further, on one side than on 
the other. He did not detect any indications of a canal-system in this shell ; nor does 
he mention that division of the septa into two laminse, one belonging to each of the con- 
tiguous chambers, which usually goes along with the other characters of this type. 
166. Organization. — My own inquiries have been chiefiy made upon a set of speci- 
mens contained in Mr. Cuming’s Philippine Collection, which present this type in a 
condition of far higher development than it has been elsewhere seen to attain ; but I 
have also had the advantage of examining large numbers of specimens from New Hol- 
land, Tenerifie, and various parts of the Indian Ocean, as well as (through the kindness 
of Mr. W. K. Paekee) numerous fossil specimens from tertiary deposits in difierent parts 
of the globe. Of these last, nearly all seem to belong to the same species, A. gibbosa, as 
that described by Professor Williamson ; and it is remarkable, that whilst the average dia- 
* Cours Elementaire de Paleontologie et de G^ologie, tom. ii. p. 201. 
t Eoraminif eres Fossiles de Yienne, p. 199. t Op. cit. p. 201. 
§ “ On tlie Minute Structure of the Calcareous Shells of some Eecent Species of Foraminifera,” in Trans- 
actions of Microscopical Society, First Series, vol. iii. p. 105. 
