GENUS AMPHISTEGINA: HISTOET. 31 



forms of the order Helicostegues, it was considered by M. D'OBBIGNY 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 saysf , 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 out, 

 that in placing Heterostegina in this order, M. D'ORBIGNY 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 

 1'autre, separees, interieurement, par des cloisons longitudinales $." 



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 Amphistegina gibbosa, a species which seems to be pretty generally diffused 

 through the tropical ocean, and which has been recently dredged up in great abundance 

 by Mr. M c ANDREW in the neighbourhood of Teneriffe. Professor WILLIAMSON showed 

 by means of horizontal and vertical sections that Amphistegina has the general structure 

 of Nummulites, but with this marked difference (as he considered), that the shell is 

 inequilateral; the spire making its convolutions obliquely instead of revolving in the 

 same plane (in other words, being turbinoid 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 chiefly 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, Teneriffe, 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 different 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 ^lementaire de Paldontologie et de Geologic, torn. ii. p. 201. 

 t Foraminiferes Fossiles de Vienne, p. 199. J Op. cit. p. 201. 



" On the Minute Structure of the Calcareous Shells of some Recent Speciea of Foraminifera," in Trans- 

 actions of Microscopical Society, First Series, vol. iii. p. 105. 



