98 FAMILY MILIOLIDA. 



when we come to examine into its value in the latter type, we shall there find it to be a mere 

 varietal or developmental diversity, it cannot here be accepted as of higher value. 



143. Affinities. — The genus we have been considering occupies a position precisely 

 intermediate between PeneropKs and Orbitolifes, to both which types it is most intimately 

 related. As already mentioned, young specimens of Orhiadina are often to be met with 

 which bear a very strong external resemblance to Pcnerojilis ; and even the absence in the 

 former of the striation usually characteristic of the latter does not furnish an absolute ground 

 of differentiation, since the striation is not unfrequently obsolete in Peneroplis (f 122). In 

 ordinary cases, however, the difference becomes evident so soon as we examine into the inter- 

 nal structure of Orliculina ; being marked by the subdivision of the principal chambers into 

 " chamberlets " by the partitions which extend transversely between the septa. It is a fact 

 of not a little significance, however, that those partitions are sometimes wanting, not merely 

 in feebly developed peneropliform varieties, but even in good-sized adunciform specimens ; 

 so that the chambers are formed upon the model of those of Peneroplis, and must be occu- 

 pied, as in that genus, by transversely elongated lobes of sarcode, instead of by a monUiform 

 series of sub-segments. In such cases, therefore, no absolute line of demarcation can be laid 

 down between Peneroplis and Orbicidina ; for, although there may be practically little or no 

 difliculty in referring any given specimen to one or the other type, by the aggregate of the 

 characters it presents, yet no one of these characters taken by itself is sufficiently constant 

 to serve as the basis for a precise definition. So, on the other side, the resemblance between 

 the peripheral portion of an OrbicuKna that has taken on the cyclical growth, and the 

 corresponding portion of those varieties of Orbitolites in which the superficial chambers are 

 not differentiated from those of the intermediate stratum (^ 173), is so extremely close, as 

 well in internal structure as in external aspect, that the two could not be distinguished by 

 anv character or combination of characters. It is, in fact, only in their early mode of growth 

 that Orbicidina axidi Orbitolites essentially differ from each other; the former being always 

 spiral, whilst the latter is ti/jjicalli/ cjcYicdX from its commencement. But we shall see (^ 180) 

 that the early growth of Orbitolites is often spiral ; and the difference between the two types 

 seems to resolve itself essentially into this, that the spiral mods of growth gives place to the 

 cyclical in Orbitolites before a second convolution is formed, so that the primordial chamber 

 and the first convolution are never invested by subsequent growths. 



144. Geo(/rapJtical Distribution. — This generic t3^pe appears to be limited to the warmer 

 parts of the ocean. It is found in great abundance near the shores of the West Indian 

 Islands, also near those of the islands of the East Indian and Polynesian Seas, being especially 

 large and abundant in the neighbourhood of the Philippine Islands. It occurs also in the Red 

 Sea, and is asserted to present itself in some parts of the Mediterranean and in the ^gean, 

 though it is certainly not a common inhabitant of those seas. So far as is at present 

 known, Orbicidina is entirely deficient in the seas of colder latitudes. 



145. Geological Distribution. — We have at present no certain knowledge of the existence 

 of Orbicidina in any formation anterior to the Tertiary series ; but in several members of this 

 it is very abundant. A fossil form closely resembling that of the Malabar Limestone has been 



