MILIOLIDA. 17 
Genus—PrnEropuis, De Montfort. 
Navtitus, Forskal, Spengler, Linné, Gmelin, Batsch, Fichtel and Moll. 
SPrroLina and CrisTELLaRIA, Lamarck. 
PrenERopLis, De Montfort, De Blainville, D’ Orbigny, Carpenter, &c. 
General characters.—Shell free, equilateral, regular, more or less nautiloid. Form 
very variable ; lenticular, outspread, or crozier-shaped. Surface usually obliquely striated. 
Hach convolution formed of numerous narrow undivided segments. The outer whorl 
embracing those within it, and in the complanate varieties almost concealing them. 
Apertures variable, either single (in young shells) or numerous and distinct, or else 
taking the form of one large dendritic orifice caused by the coalescing of a linear series 
of pores. 
Subgenus—Denpritina, D’ Orbigny. 
General characters.—Shell nautiloid, lenticular, turgid. Pseudopodial aperture large, 
irregular, dendritic. 
1. Denpritina arBuscuLa, D’ Orbigny. Plate III, figs. 48, 49. 
DrnvRITINA ARBUSCULA, D’Oré., 1826. Modéle No.21. Ann. de Sc. Nat., vol. vii, p. 285, 
No. 1, pl. 15, figs. 6, 7. 
PENEROPLIS PLANATUS (F'. and M.), var., Carpenter, 1859. Phil. Trans., vol. cxlix, p. 9, 
pl. 2; Introd. For., p. 88, pl. 8. 
Characters.—Shell nautiloid, turgid, thickened at the umbilicus, rounded more or less 
at the margin. Aperture a single large ramifying orifice, formed by the coalescence of 
numerous small pores, arranged either in a line or otherwise. Diameter, 4th inch. 
In speaking of the earlier authors who have studied the different forms of Peneroplis - 
(‘ Ann. Nat. Hist.,’ March, 1865), we have stated our views fully as to the value of the 
subdivision of the type into genera and species. (See also Carpenter’s memoir, ‘ Phil. 
Trans.,’ 1859, and his ‘Introd. Foram.,’ p. 84.) Notwithstanding the wide variations in 
general contour, and in the nature of the pseudopodial apertures which may be observed 
in different specimens, there can be no doubt that the whole constitute but one true 
species. At the same time we are able to divide them roughly, according to the nature 
of their divergence from the central type, into three or four groups, for which, as causing 
least confusion, we propose to keep the well-known and hitherto accepted names, giving 
to them a subgeneric place. Of these groups, that centering in Pexeroplis (Dendritina) 
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