GENUS OPEECTJLINA : HISTOET. 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'ORBIGNY himself, as well as by MM. D'ARCHIAC and HAIME, and to the 

 recent type described as Operculina by Mr. CARTER. 



140. History. The genus Operculina 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 differing 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 de 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 derniere formant saillie de toute la largeur de la spire, a tous les ages percee 

 d'une ouverture visible triangulaire, centre le retour de la spire." In his ' Cours Ele- 

 mentaire de Paleontologie ' (torn. 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'ORBIGNY'S definitions of three other genera nearly allied to Operculina ; 

 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. 

 Nonionina. " Coquille nautiloide, pourvue d'une seule ouverture en fente transversale, 

 non masquee, placee contre le retour de la spire." Nummulites. " Coquille lenticulaire 

 a tours embrassants, pourvue d'une ouverture en fente transversale, souvent masquee, 

 contre le retour de la spire." Assilina. " Ce sont des. Nummulites dont les tours de la 

 spire, ne rejoignant pas le centre, sont tous 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 differentes, a tours plus ou moins serres, 

 plus ou moins embrassants, quelquefois simplement juxtaposes. Dans le jeune age, le 

 dernier est saillant ; plus tard, les tours se rapprochent de telle sorte qu'a 1'etat adulte 

 le dernier parait se souder tout a fait a 1'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 Tamma!" (p. 70). The genus Nonionina is said by these 



* Description des Animaux Fossiles du Groupe Nummulitique de PInde. Paris, 1853. 



