GENUS OPEECULINA -HISTORY. 
13 
in general aspect and particular features, yet having so many structural characters in 
common as to be obviously referrible to the same generic type. This type I shall 
designate, provisionally at least, as Operculina ; since, although it does not correspond 
with the definition of this genus as given by its founder M. d’Orbigny, it is obviously 
related most closely to the fossil forms which have been described under that designa- 
tion by M. d’Orbignt himself, as well as by MM. d’Archiac and Haime, and to the 
recent type described as OpercuUna by Mr. Carter. 
140. Histm'y. — ^The genus OpercuUna was created by M. d’Orbigny in 1825, for the 
reception of a group of Foraminifera bearing a strong general resemblance to Nummulites^ 
but considered by him as difiering from it in the extreme compression of its discoidal 
shell, in the smaller number of its whorls, in the non-investment of the earlier whorls 
by the later, so that the former remain apparent on both sides through the whole of 
life, and also in the form of the aperture, which is a transverse slit in Nummulites, but 
triangular in Operculina. The following is the definition of this genus given by him in 
his ‘ Foraminiferes Fossiles (Je Vienne ’ (p. 117) : — “ Coquille libre, equilaterale, ovale ou 
discoidale, tres comprimee, formee d’une spire non embrassante, reguliere, egalement 
apparent de chaque cote, a tours contigus, et croisant tres rapidement. Loges nombreuses, 
etroites, la demiere formant saillie de toute la largeur de la spire, a tons les ages percee 
d’une ouverture visible tiiangulaire, centre le retour de la spire.” In his ‘ Cours Fle- 
mentaii-e de Paleontologie ’ (tom. ii. p. 197), he defines the genus by the following 
characters alone: — “Coquille comprimee a tours apparents, pourvue d’une ouverture 
triangulaire centre le retour de la spire.” I think it necessary also to quote from the 
same work M. d’Orbigxy’s definitions of three other genera nearly allied to Opercuhna ; 
since, as will presently appear, the characters which he assigns to them are based on an 
entirely wrong idea of the distinctive features presented by their respective types. - 
Nmionina. “ Coquille nautiloide, poui’\-ue d’une seule ouverture en fente transversale, 
non masquee, placee contre le retour de la spire. — Nu'i:n'i7iulit€s. “ Coquille lenticulaiie 
a tours embrassants, pomwue d’une ouverture en fente transversale, souvent masquee, 
contre le retour de la sphe.” — Asdlina. “ Ce sont des Nummulites dont les tours de la 
spire, ne rejoignant pas le centre, sont tons plus ou moins apparents.” In the admi- 
rable Monograph of MM. d’Archiac and Haime the genus Nummulites is defined as 
follows: — “Coquille libre, symmetrique, orbiculaire, plane, discoide, lenticulaire ou 
subglobuleuse, composee d’une lame calcaire pliee et enroulee suivant un meme plan, 
criblee de pores ou canaux de trois grandeurs difierentes, a tours plus ou moins serres, 
plus ou moins embrassants, quelquefois simplement juxtaposes. Hans le jeune age, le 
dernier est saillant ; plus tard, les tours se rapprochent de telle sorte qu’a I’etat adulte 
le dernier parait se souder tout a fait a I’avant dernier. Canal spiral divise par des 
cloisons transverses, plus ou moins nombreuses, percees a leur base, contre le retour de 
la spire, d’une ouverture lineaire transverse. Ces derniers caracteres semblent s’etre 
attenues vers la fin de la vie de I’animal” (p. 70). The genus Nonionina is said by these 
* Description des Animaux Eossiles du Groupe Nummulitique de ITnde. Paris, 1853, 
