FORAMINIFERA OF THE NORTH PACIFIC OCEAN. 23 



number of genera which take on uniserial or broadly flaring later 

 developmental stages, but still keep to a single plane. 



The more specialized group represented by Quinqueloculina as a 

 basal member after the second Cornu spira-like stage develops cham- 

 bers in constantly changing but definitely related planes. The 

 earliest of these as seen in the ontogeny of various forms is quinque- 

 loculine, chambers added in planes successively 144° apart and taking 

 two horizontal whorls to complete a cycle. Following this in the 

 ontogeny is a triloculine stage in higher genera with planes 120° apart, 

 followed in some genera by a biloculine stage with planes 180° apart, 

 and this followed in the highest genera by a uniloculine stage where 

 chambers almost entirely include the previously developed cham- 

 bers. These stages are especially complete in the microspheric forms 

 of the species. The series of stages quinqueloculine, triloculine, 

 biloculine, uniloculine is never, so far as has been seen, in any way 

 reversed, and is only changed by the dropping out of stages in the 

 accelerated megalospheric form. All of the four stages above men- 

 tioned are represented by generic types, the species of which show 

 both microspheric and megalospheric forms. 



Much of the apparent "variation" in this group may be directly 

 traced to the differences in the microspheric and megalospheric forms 

 and the various stages in development represented by the material 

 at the time of collecting, especially if living tests are collected. By 

 a full series of specimens and careful sectioning it should be possible 

 to much more clearly define specific limits of true variation and a 

 realization of the stages of development in the two forms of the 

 species, a refinement winch has not hitherto been attempted in any 

 1aj*cta scries 



SYSTEMATIC TREATMENT. 



Family 10. MILIOLIDAE. 



Test typically calcareous, imperforate except in the very early 

 stages of certain genera, porcellanous ; sometimes the exterior with 

 arenaceous covering, but always on an imperforate calcareous base, 

 aperture typically with a tooth variously modified in different genera. 



This family is one with a similar young throughout the group in 

 the very early stages of at least the microspheric form of each species. 

 The greatest number of species and individuals occur in the shallow 

 waters of tropical seas, where, as in the West Indies, certain species 

 such as Orbiculina adunca make up a large percentage of the forami- 

 nifcral content of shallow-water dredgings. The various species of 

 Orbitolites are among the largest of the living foraminifera and are 

 a conspicuous factor of the shallow water dredgings of certain parts 

 of the Indo-Pacific region especially. The members of the family, 

 however, are by no means limited to shallow tropical seas, as certain 



